Don't search for nested repositories during git-gc

The git-gc script searches for all repositories in the git-directory
of the site. However, it would do that recursively for all sub
directories, i.e. within the repositories. This is unnecessary
and for large repositories this could take minutes.

Change-Id: I3747df4da7dd45cd927527ca88de25656307eda7
diff --git a/container-images/git-gc/tools/gc.sh b/container-images/git-gc/tools/gc.sh
index 6dfd665..2b1c17c 100755
--- a/container-images/git-gc/tools/gc.sh
+++ b/container-images/git-gc/tools/gc.sh
@@ -93,7 +93,7 @@
 
 gc_all_projects()
 {
-  for d in $(cd $TOP && find . -type d -name "*.git" | cut -c3- -)
+  for d in $(cd $TOP && find . -type d -path "*.git" -prune -o -name "*.git" | cut -c3- -)
   do
     gc_project "$d"
   done