parse-remote: handle detached HEAD

get_remote_merge_branch with zero or one arguments returns the
upstream branch. But a detached HEAD does no have an upstream branch,
as it is not tracking anything. Handle this case testing the exit code
of "git symbolic-ref -q HEAD".

Reported-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Santi Béjar <santi@agolina.net>
Tested-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-parse-remote.sh

@@ -66,7 +66,7 @@ get_remote_merge_branch () {
 	    origin="$1"
 	    default=$(get_default_remote)
 	    test -z "$origin" && origin=$default
-	    curr_branch=$(git symbolic-ref -q HEAD)
+	    curr_branch=$(git symbolic-ref -q HEAD) &&
 	    [ "$origin" = "$default" ] &&
 	    echo $(git for-each-ref --format='%(upstream)' $curr_branch)
 	    ;;