Update internal usage to use the `git_reference` names for constants.

Use the new object_type enumeration names within the codebase.

Windows defines `timeval` with `long`, which we cannot
sanely cope with.  Instead, use a custom timeval struct.

When ticking over one second, it can happen that the actual time ticks
over the same second between the time that we undermine our own race
protections and the time in which we perform the index update. Such
timing would make the time in the entries match the index' timestamp and
we have not gained anything.

Ticking over five seconds makes it so that if real-time rolls over that
second, our index is still ahead. This is still suboptimal as we're
dealing with timing, but five seconds should be long enough for any
reasonable test runner to finish the tests.

Provide functionality to set the time on a filesystem entry,
using utimes or futimes on POSIX type systems or SetFileTime
on Win32.

These tests want to test that we don't recalculate entries which match
the index already. This is however something we force when truncating
racily-clean entries.

Tick the index forward as we know that we don't perform the
modifications which the racily-clean code is trying to avoid.