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.

Don't propagate workdir's mode to the index during diff's update index

When updating the index during a diff, preserve the original mode,
which prevents us from dropping the mode to what we have interpreted
as on our system (eg, what the working directory claims it to be,
which may be a lie on some systems.)

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

This is used by the submodule in order to figure out if the index has
changed since it last read it. Using a timestamp is racy, so let's make
it use the checksum, just like we now do for reloading the index itself.

Quote LIBSSH2_LIBRARIES call

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.