gnu: libgit2: Introduce ‘libgit2-1.5’ variable.

* gnu/packages/version-control.scm (libgit2): Rename to…
(libgit2-1.5): … this.
(libgit2): Alias for ‘libgit2-1.5’.

Change-Id: I7f6aff23df3b968d914736fe723dcdd435176ce2

gnu/packages/version-control.scm

@@ -874,7 +874,7 @@ which has been extracted into a standalone library for compatibility with
 other git-like projects such as @code{libgit2}.")
       (license license:lgpl2.1+))))
 
-(define-public libgit2
+(define-public libgit2-1.5
   (package
     (name "libgit2")
     (version "1.5.1")
@@ -942,6 +942,10 @@ write native speed custom Git applications in any language with bindings.")
     ;; GPLv2 with linking exception
     (license license:gpl2)))
 
+(define-public libgit2
+  ;; Default version of libgit2.
+  libgit2-1.5)
+
 (define-public libgit2-1.7
   (package
     (inherit libgit2)