git-fetch: fix dumb protocol transport to fetch from pack-pruned ref

git-fetch: fix dumb protocol transport to fetch from pack-pruned ref

Earlier, commit walkers downloaded loose refs from refs/ hierarchy
of the remote side to find where to start walking; this would
not work for a repository whose refs are packed and then pruned.

With the previous change, we have ls-remote output from the
remote in-core; we can use the value from there without
requiring loose refs anymore.

Signed-off-by: Junio C Hamano <junkio@cox.net>