gnu: Add emacs-libgit.

* gnu/packages/emacs-xyz.scm (emacs-libgit): New variable.
* gnu/packages/patches/emacs-libgit-use-system-libgit2.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register patch.

gnu/local.mk

@@ -19,7 +19,7 @@
 # Copyright © 2018 Amirouche Boubekki <amirouche@hypermove.net>
 # Copyright © 2018, 2019, 2020 Oleg Pykhalov <go.wigust@gmail.com>
 # Copyright © 2018 Stefan Stefanović <stefanx2ovic@gmail.com>
-# Copyright © 2018 Maxim Cournoyer <maxim.cournoyer@gmail.com>
+# Copyright © 2018, 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 # Copyright © 2019 Guillaume Le Vaillant <glv@posteo.net>
 # Copyright © 2019, 2020 John Soo <jsoo1@asu.edu>
 # Copyright © 2019 Jonathan Brielmaier <jonathan.brielmaier@web.de>
@@ -886,6 +886,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/emacs-fix-scheme-indent-function.patch	\
   %D%/packages/patches/emacs-json-reformat-fix-tests.patch	\
   %D%/packages/patches/emacs-highlight-stages-add-gexp.patch	\
+  %D%/packages/patches/emacs-libgit-use-system-libgit2.patch    \
   %D%/packages/patches/emacs-magit-log-format-author-margin.patch	\
   %D%/packages/patches/emacs-scheme-complete-scheme-r5rs-info.patch	\
   %D%/packages/patches/emacs-source-date-epoch.patch		\

gnu/packages/emacs-xyz.scm

@@ -31,7 +31,7 @@
 ;;; Copyright © 2017 Peter Mikkelsen <petermikkelsen10@gmail.com>
 ;;; Copyright © 2017, 2018, 2019, 2020 Tobias Geerinckx-Rice <me@tobias.gr>
 ;;; Copyright © 2017 Mike Gerwitz <mtg@gnu.org>
-;;; Copyright © 2017, 2018, 2019 Maxim Cournoyer <maxim.cournoyer@gmail.com>
+;;; Copyright © 2017, 2018, 2019, 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2018 Sohom Bhattacharjee <soham.bhattacharjee15@gmail.com>
 ;;; Copyright © 2018, 2019 Mathieu Lirzin <mthl@gnu.org>
 ;;; Copyright © 2018, 2019, 2020 Pierre Neidhardt <mail@ambrevar.xyz>
@@ -318,6 +318,82 @@ For remote processes a substitute is provided, which communicates with Emacs
 on stdout instead of using a socket as the Emacsclient does.")
     (license license:gpl3+)))
 
+(define-public emacs-libgit
+  (let ((commit "0ef8b13aef011a98b7da756e4f1ce3bb18e4d55a")
+        (revision "1"))
+    (package
+      (name "emacs-libgit")
+      (version (git-version "20200515" revision commit))
+      (source (origin
+                (method git-fetch)
+                (uri (git-reference
+                      (url "https://github.com/magit/libegit2.git")
+                      (commit commit)))
+                (file-name (git-file-name name version))
+                (sha256
+                 (base32
+                  "0pnjr3bg6y6354dfjjxfj0g51swzgl1fncpprah75x4k94rd369f"))
+                (patches (search-patches
+                          ;; Submitted for inclusion upstream (see:
+                          ;; https://github.com/magit/libegit2/pull/96).
+                          "emacs-libgit-use-system-libgit2.patch"))))
+      ;; Use the cmake-build-system as it provides support for cross builds.
+      (build-system cmake-build-system)
+      (arguments
+       `(#:configure-flags '("-DUSE_SYSTEM_LIBGIT2=x")
+         ;; Add the emacs-build-system byte compilation and install phases.
+         #:imported-modules (,@%cmake-build-system-modules
+                             (guix build emacs-build-system)
+                             (guix build emacs-utils))
+         #:modules ((guix build cmake-build-system)
+                    ((guix build emacs-build-system) #:prefix emacs:)
+                    (guix build emacs-utils)
+                    (guix build utils))
+         #:phases
+         (modify-phases %standard-phases
+           (add-after 'unpack 'set-libgit--module-file
+             (lambda* (#:key outputs #:allow-other-keys)
+               (let ((out (assoc-ref outputs "out")))
+                 (make-file-writable "libgit.el")
+                 (emacs-substitute-variables "libgit.el"
+                   ("libgit--module-file"
+                    (string-append out "/share/emacs/site-lisp/libegit2.so")))
+                 #t)))
+           (add-before 'install 'prepare-for-install
+             (lambda _
+               (let ((s (string-append "../" ,name "-" ,version "-checkout")))
+                 (copy-file "libegit2.so" (string-append s "/libegit2.so"))
+                 (chdir s)
+                 #t)))
+           (replace 'install
+             (lambda* (#:key outputs #:allow-other-keys)
+               (let ((install (assoc-ref emacs:%standard-phases 'install)))
+                 (install #:outputs outputs
+                          #:include (cons "\\.so$"
+                                          emacs:%default-include)))))
+           (add-after 'install 'make-autoloads
+             (assoc-ref emacs:%standard-phases 'make-autoloads))
+           (add-after 'make-autoloads 'enable-autoloads-compilation
+             (assoc-ref emacs:%standard-phases 'enable-autoloads-compilation))
+           (add-after 'enable-autoloads-compilation 'patch-el-files
+             (assoc-ref emacs:%standard-phases 'patch-el-files))
+           (add-after 'patch-el-files 'emacs-build
+             (assoc-ref emacs:%standard-phases 'build))
+           (add-after 'emacs-build 'validate-compiled-autoloads
+             (assoc-ref emacs:%standard-phases 'validate-compiled-autoloads)))))
+      (native-inputs
+       `(("pkg-config" ,pkg-config)
+         ("emacs" ,emacs-no-x)
+         ("git" ,git-minimal)))
+      (inputs
+       `(("libgit2" ,libgit2)))
+      (home-page "https://github.com/magit/libegit2")
+      (synopsis "Emacs bindings for libgit2")
+      (description "This is an experimental module written in C providing
+libgit2 bindings for Emacs, intended to boost the performance of Magit.")
+      ;; The LICENSE file says GPL v2+, but libgit.el says GPL v3+.
+      (license license:gpl3+))))
+
 (define-public emacs-magit
   ;; `magit-setup-buffer' macro introduced in c761d28d and required in
   ;; `emacs-forge'.

gnu/packages/patches/emacs-libgit-use-system-libgit2.patch

@@ -0,0 +1,88 @@
+From de3c48d72ec7064e7f0522877fe759c729df0c50 Mon Sep 17 00:00:00 2001
+From: Maxim Cournoyer <maxim.cournoyer@gmail.com>
+Date: Wed, 25 Mar 2020 11:32:18 -0400
+Subject: [PATCH] Allow using a system provided libgit2 library
+
+Setting the USE_SYSTEM_LIBGIT2 Make or CMake variable (through the
+BUILD_OPTIONS variable) to any value enables using the system library.
+The default behavior of using a bundled copy of libgit2 is unchanged.
+---
+ CMakeLists.txt     |  9 +++++++--
+ Makefile           | 11 +++++++++++
+ src/CMakeLists.txt |  9 +++++++--
+ 3 files changed, 25 insertions(+), 4 deletions(-)
+
+diff --git a/CMakeLists.txt b/CMakeLists.txt
+index a393d7c..75d6ca6 100644
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -7,9 +7,14 @@ set(BUILD_SHARED_LIBS OFF CACHE BOOL "shared" FORCE)
+ set(BUILD_CLAR OFF CACHE BOOL "clar" FORCE)
+ set(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -DEGIT_DEBUG")
+ 
+-add_subdirectory(libgit2)
++if(USE_SYSTEM_LIBGIT2)
++  find_package(PkgConfig REQUIRED)
++  pkg_check_modules(git2 REQUIRED IMPORTED_TARGET libgit2)
++else()
++  add_subdirectory(libgit2)
++  find_library(git2 libgit2.a)
++endif()
+ 
+-find_library(git2 libgit2.a)
+ add_subdirectory(src)
+ 
+ enable_testing()
+diff --git a/Makefile b/Makefile
+index 8199532..6a6a4e1 100644
+--- a/Makefile
++++ b/Makefile
+@@ -13,6 +13,13 @@ ifeq ($(UNAME),MSYS)
+ 	BUILD_OPTIONS+= -G "MSYS Makefiles"
+ endif
+ 
++# If the variable USE_SYSTEM_LIBGIT2 is set to *any* value, use the
++# system provided libgit2 library.
++USE_SYSTEM_LIBGIT2? := \
++	$(if $(or $(USE_SYSTEM_LIBGIT2),\
++	 	  $(findstring USE_SYSTEM_LIBGIT2,$(BUILD_OPTIONS))),\
++		true)
++
+ ifeq "$(TRAVIS)" "true"
+ ## Makefile for Travis ###################################################
+ #
+@@ -87,7 +94,11 @@ submodule-update:
+ 	@git submodule update
+ 
+ libgit2:
++ifeq ($(USE_SYSTEM_LIBGIT2?),)
+ 	@git submodule update --init
++else
++	@echo "Using the system provided libgit2 library"
++endif
+ 
+ CLEAN  = $(ELCS) $(PKG)-autoloads.el build
+ 
+diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
+index cfb5777..0dbad8a 100644
+--- a/src/CMakeLists.txt
++++ b/src/CMakeLists.txt
+@@ -13,8 +13,13 @@ if(WIN32)
+   set_target_properties(egit2 PROPERTIES PREFIX lib)
+ endif(WIN32)
+ 
+-target_link_libraries(egit2 git2)
+-target_include_directories(egit2 SYSTEM PRIVATE "${libgit2_SOURCE_DIR}/include")
++if(USE_SYSTEM_LIBGIT2)
++  target_link_libraries(egit2 PRIVATE PkgConfig::git2)
++else()
++  target_link_libraries(egit2 git2)
++  target_include_directories(
++    egit2 SYSTEM PRIVATE "${libgit2_SOURCE_DIR}/include")
++endif()
+ 
+ if(CMAKE_COMPILER_IS_GNUCC)
+   target_compile_options(egit2 PRIVATE -Wall -Wextra)
+-- 
+2.26.2
+