ci: Silence some warnings

... as suggested by Jonas in mutter!548.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/509

.gitlab-ci/checkout-mutter.sh

@@ -17,7 +17,7 @@ if [ "$CI_MERGE_REQUEST_TARGET_BRANCH_NAME" ]; then
   merge_request_branch=$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME
 
   echo Looking for $merge_request_branch on remote ...
-  if git fetch $merge_request_remote $merge_request_branch >/dev/null 2>&1; then
+  if git fetch -q $merge_request_remote $merge_request_branch; then
     mutter_target=FETCH_HEAD
   fi
 fi
@@ -28,4 +28,4 @@ if [ -z "$mutter_target" ]; then
   echo Using $mutter_target instead
 fi
 
-git checkout $mutter_target
+git checkout -q $mutter_target