git-am: Run git gc only once and not for every patch.

With "too many unreachable loose objects" git gc --auto will always
trigger. This clutters the output of git am and thus git rebase.

Signed-off-by: Michael Stefaniuc <mstefani@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -469,9 +469,9 @@ do
 		"$GIT_DIR"/hooks/post-applypatch
 	fi
 
-	git gc --auto
-
 	go_next
 done
 
+git gc --auto
+
 rm -fr "$dotest"