gitweb: use new Git::Repo API, and add optional caching

gitweb: use new Git::Repo API, and add optional caching

Gitweb now uses the Git::Repo API; this change is behavior-preserving,
except for slightly more aggressive error handling; see below.

This patch also adds an optional caching layer for caching repository
data using any Cache::Cache-compatible caching engine (typically
Cache::Memcached).  Larger cacheable items, like blobs, snapshots or
diffs, can be cached on disk.

Other minor changes:

- Gitweb would previously accept invalid input and either (a) display
  nothing, (b) display an obscure error message, or (c) proceed as
  normal since the parameter happens to be unused in the particular
  code path used.  This has changed in that gitweb will check for
  parameter correctness more aggressively, and display meaningful
  error messages.  This change is only relevant if you manually edit
  gitweb's CGI parameters, since gitweb only generates valid links.

- Empty projects:

  - Only display summary link for empty projects in project list to
    avoid broken links (yielding 404).

  - Slim down summary page for empty projects to avoid some broken
    links and unnecessary vertical space.

  - Sort empty projects at the bottom of the project list when sorting
    by last change.

  - Add test for empty projects to t9503 (the Mechanize test), now
    that there no broken links anymore.

Signed-off-by: Lea Wiemann <LeWiemann@gmail.com>