gitweb: Use git-show-ref instead of git-peek-remote

gitweb: Use git-show-ref instead of git-peek-remote

Use "git show-ref --dereference" instead of "git peek-remote
$projectroot/project" in git_get_references. git-show-ref is faster
than git-peek-remote (40ms vs 56ms user+sys for git.git repository);
even faster is reading info/refs file (if it exists), but the
information in info/refs can be stale; that and the fact that
info/refs is meant for dumb protocol transports, not for gitweb.

git-show-ref is available since v1.4.4; the output format is slightly
different than git-peek-remote output format, but we accept both.

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