completion: silence "fatal: Not a git repository" error

It is possible that a user is trying to run a git command and fail
to realize that they are not in a git repository or working tree.
When trying to complete an operation, __git_refs would fall to a
degenerate case and attempt to use "git for-each-ref", which would
emit the error.

Hide this error message coming from "git for-each-ref".

Signed-off-by: John Szakmeister <john@szakmeister.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -388,7 +388,8 @@ __git_refs ()
 		;;
 	*)
 		echo "HEAD"
-		git for-each-ref --format="%(refname:short)" -- "refs/remotes/$dir/" | sed -e "s#^$dir/##"
+		git for-each-ref --format="%(refname:short)" -- \
+			"refs/remotes/$dir/" 2>/dev/null | sed -e "s#^$dir/##"
 		;;
 	esac
 }