channels: Autoload (git …) modules.

Autoloading Guile-Git is important in cases where (guix channels) is
used for little more than the <channel> definition.  This is the case,
for example, of ‘guix describe’ or ‘guix shell’.

This reduces from 177 to 121 the number of .go files loaded when
running:

  ./pre-inst-env strace -e openat -o /tmp/log.strace \
     guix describe -p /var/guix/profiles/per-user/$USER/current-guix
  grep 'openat.*\.go.* = [0-9]' < /tmp/log.strace |wc -l

Likewise, it reduces the max RSS (as measured by ‘time -f %M guix
describe -p …’) from 54 to 37 MiB.

* guix/channels.scm: Autoload (git …) modules.

Change-Id: Ia58a99c865bf0f6fe461a1e71390d075e760f8d6

guix/channels.scm

@@ -20,7 +20,13 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix channels)
-  #:use-module (git)                              ;TODO: autoload
+  #:autoload   (git commit) (commit-lookup
+                             commit-id)
+  #:autoload   (git oid) (oid->string
+                          string->oid)
+  #:autoload   (git object) (object-id)
+  #:autoload   (git errors) (GIT_ENOTFOUND)
+  #:autoload   (git structs) (git-error-code)
   #:autoload   (guix git) (update-cached-checkout
                            url+commit->name
                            commit-difference