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: NW Trevor King <wking@drexel.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>