gitweb: support lastactivity file

gitweb: support lastactivity file

Provide a new gitweb configuration value for $lastactivity_file
which is the pathname relative to a $GIT_DIR for a file that if
it exists and is non-empty and contains a date in iso, iso-strict
or raw format will be read and used as the value returned by the
git_get_last_activity function.

This is most useful if a hooks/post-update script is present which
contains something like this:

  git for-each-ref --sort=-committerdate --format='%(committerdate:iso8601)' \
                   --count=1 refs/heads > info/lastactivity

And then the gitweb_config.perl configuration contains this:

  our $lastactivity_file = "info/lastactivity";

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