GitTimeParser: A date parser using the java.time API

Replacement of GitDateParser that uses java.time classes instead of
the obsolete Date. Updating GitDateParser would have been a mess of
deprecation and methods with confusing names, so I opted for writing a
parallel class with the new types.

Some differences:

* The new DateTimeFormatter is thread-safe, so we don't need the
LocalThread cache

* No code seems to use other locale than the default, we don't need to
cache per locale either

Change-Id: If24610a055a47702fb5b7be2fc35a7c722480ee3