git: Honor proxy settings when fetching submodules.

Fixes <https://bugs.gnu.org/44593>.

* guix/git.scm (update-submodules): Add #:fetch-options and honor it.
(update-cached-checkout): Pass #:fetch-options to 'update-submodules'.
* doc/guix.texi (Requirements): Adjust comment about Guile-Git.

doc/guix.texi

@@ -848,7 +848,8 @@ version 0.1.0 or later;
 @item @uref{https://notabug.org/guile-lzlib/guile-lzlib, Guile-lzlib};
 @item @uref{https://www.nongnu.org/guile-avahi/, Guile-Avahi};
 @item
-@c FIXME: Specify a version number once a release has been made.
+@c FIXME: We need the #:fetch-options parameter of 'submodule-update',
+@c which appeared in 0.5.0.  Change below after string freeze.
 @uref{https://gitlab.com/guile-git/guile-git, Guile-Git}, version 0.3.0
 or later;
 @item @uref{https://savannah.nongnu.org/projects/guile-json/, Guile-JSON}

guix/git.scm

@@ -283,13 +283,15 @@ dynamic extent of EXP."
       (report-git-error err))))
 
 (define* (update-submodules repository
-                            #:key (log-port (current-error-port)))
+                            #:key (log-port (current-error-port))
+                            (fetch-options #f))
   "Update the submodules of REPOSITORY, a Git repository object."
   (for-each (lambda (name)
               (let ((submodule (submodule-lookup repository name)))
                 (format log-port (G_ "updating submodule '~a'...~%")
                         name)
-                (submodule-update submodule)
+                (submodule-update submodule
+                                  #:fetch-options fetch-options)
 
                 ;; Recurse in SUBMODULE.
                 (let ((directory (string-append
@@ -297,6 +299,7 @@ dynamic extent of EXP."
                                   "/" (submodule-path submodule))))
                   (with-repository directory repository
                     (update-submodules repository
+                                       #:fetch-options fetch-options
                                        #:log-port log-port)))))
             (repository-submodules repository)))
 
@@ -397,7 +400,8 @@ it unchanged."
        (remote-fetch (remote-lookup repository "origin")
                      #:fetch-options (make-default-fetch-options)))
      (when recursive?
-       (update-submodules repository #:log-port log-port))
+       (update-submodules repository #:log-port log-port
+                          #:fetch-options (make-default-fetch-options)))
 
      ;; Note: call 'commit-relation' from here because it's more efficient
      ;; than letting users re-open the checkout later on.