gitweb: Do not parse refs by hand, use git-peek-remote instead

gitweb: Do not parse refs by hand, use git-peek-remote instead

This is in response to Linus's work on packed refs. Additionally it
makes gitweb work with symrefs, too.

Do not parse refs by hand, using File::Find and reading individual
heads to get hash of reference, but use git-peek-remote output
instead.  Assume that the hash for deref (with ^{}) always follows hash
for ref, and that we have derefs only for tag objects; this removes
call to git_get_type (and git-cat-file -t invocation) for tags, which
speeds "summary" and "tags" views generation, but might slow generation
of "heads" view a bit.  For now, we do not save and use the deref hash.

Remove git_get_hash_by_ref while at it, as git_get_refs_list was the
only place it was used.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>