Fix Repository#rev_list silently ignoring :since and :until when they're of a class other than Time

Currently, the `:since` and `:until` arguments to `Grit::GitRuby::Repository#rev_list` must be `is_a?(Time)`, or they're silently ignored. This is confusing, so this patch simply removes the check and adds a test to verify that a `Time`-like object is accepted.