gitweb: add If-Modified-Since handling to git_snapshot().
Because snapshots can be large, you can save some bandwidth by
supporting caching via If-Modified-Since. This patch adds support for
the i-m-s request to git_snapshot() if the request is a commit.
Requests for snapshots of trees, which lack well defined timestamps,
are still handled as they were before.
Signed-off-by: W Trevor King <wking@drexel.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>