This allows the user to look up fields which we don't parse in libgit2,
and allows them to access gpgsig or mergetag fields if they wish to
check the signature.

Test to ensure that when status updates an index, it does not alter
the original mode for file types that are not supported (eg, symlinks
on Windows).

Fixed index being double-freed in stash tests

Use the checksum to check whether an index has been modified

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.

This will tell us which numbers we were trying to compare, rather than
just telling us that they're different.

Zero out racily-clean entries' file_size