This diff does two things:

1. aligns path_cache behavior in determinator.rb so that both formatted and raw
calls to last_modified_time are read from the path_cache

2. adds option `use-git-cache` which improves render performance by reading the
entire git log once (instead of 1:1 for each file)

`item.path` returns the absolute path to a file, whereas other instantiations
of `Determinator`, like in `tag.rb`, use a relative path. by using a relative
path in both places we increase the likelihood of cache hits.