completion: avoid ls-remote in certain scenarios

It's _very_ slow in many cases, and there's really no point in fetching
*everything* from the remote just for completion. In many cases it might
be faster for the user to type the whole thing.

If the user manually specifies 'refs/*', then the full ls-remote
completion is triggered.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -427,14 +427,8 @@ __git_refs ()
 		done
 		;;
 	*)
-		git ls-remote "$dir" HEAD ORIG_HEAD 'refs/tags/*' 'refs/heads/*' 'refs/remotes/*' 2>/dev/null | \
-		while read -r hash i; do
-			case "$i" in
-			*^{}) ;;
-			refs/*) echo "${i#refs/*/}" ;;
-			*) echo "$i" ;;
-			esac
-		done
+		echo "HEAD"
+		git for-each-ref --format="%(refname:short)" -- "refs/remotes/$dir/" | sed -e "s#^$dir/##"
 		;;
 	esac
 }