Update unit test to use newfangled `st_ctime_nsec`, which provides
indirection to the platform-correct name.

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

This allows us to remove OS checks from source code, instead relying
on CMake to detect whether or not `struct stat` has the nanoseconds
members we rely on.

As we attempt to replicate a situation in which an older checkout has
put a file on disk with different filtering settings from us, set the
timestamp on the entry and file to a second before we're performing the
operation so the entry in the index counts as old.

This way we can test that we're not looking at the on-disk file when the
index has the entry and we detect it as clean.

This function is meant to simulate what git does in the reset command,
so we should include the reflog message in that.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

Rename git_merge_head to git_annotated_commit, as it becomes used
in more operations than just merge.