[PATCH 3/3] gitweb: File based caching layer (from git.kernel.org)

This is a relatively large patch that implements the file based
caching layer that is quite similar to the one used  on such large
sites as kernel.org and soon git.fedoraproject.org.  This provides
a simple, and straight forward caching mechanism that scales
dramatically better than Gitweb by itself.

The caching layer basically buffers the output that Gitweb would
normally return, and saves that output to a cache file on the local
disk.  When the file is requested it attempts to gain a shared lock
on the cache file and cat it out to the client.  Should an exclusive
lock be on a file (it's being updated) the code has a choice to either
update in the background and go ahead and show the stale page while
update is being performed, or stall the client(s) until the page
is generated.

There are two forms of stalling involved here, background building
and non-background building, both of which are discussed in the
configuration page.

There are still a few known "issues" with respect to this:
- Code needs to be added to be "browser" aware so
  that clients like wget that are trying to get a
  binary blob don't obtain a "Generating..." page

Caching is disabled by default with the $cache_enable variable,
setting this to 1 will enable file based caching.  It is expected
that this will be extended to include additional types of caching
(like memcached) in the future and should not be exclusively
considered a binary value.
---
 gitweb/cache.pm          |  365 ++++++++++++++++++++++++++++++++++++++++++++++
 gitweb/gitweb.perl       |   99 +++++++++++--
 gitweb/static/gitweb.css |    6 +
 3 files changed, 458 insertions(+), 12 deletions(-)
 create mode 100644 gitweb/cache.pm