gitweb: support html cache of summary output

gitweb: support html cache of summary output

The information on the summary page can be expensive to generate.

If the information has not changed then it's much faster to
simply provide the cache of the page output rather than spending
the time to regenerate exactly the same information again.

Since the summary page is typically the first page visited for
a project (and also one of the more expensive ones to generate)
it makes sense to support caching it as its load speed is likely
to be highly visible to the end user.

Provide configuration variables to specify the caching directory
(relative to $GIT_DIR) and whether or not to use a cache for
the summary page.

In order for this to be useful, a post-update hook will be
needed to remove any summary cache so that the page will be
regenerated when appropriate.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>