remote: create fetch config lines with '+'

Since git-remote always uses remote tracking branches, it
should be safe to always force updates of those branches.
I.e., we should generate

  fetch = +refs/heads/*:refs/remotes/$remote/*

instead of

  fetch = refs/heads/*:refs/remotes/$remote/*

This was the behavior of the perl version, which seems to
have been lost in the C rewrite.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-remote.c

@@ -105,6 +105,7 @@ static int add(int argc, const char **argv)
 		struct path_list_item *item = track.items + i;
 
 		strbuf_reset(&buf2);
+		strbuf_addch(&buf2, '+');
 		if (mirror)
 			strbuf_addf(&buf2, "refs/%s:refs/%s",
 					item->path, item->path);