We test the generation of the textual patch via the patch function,
which are just one of two possibilities to get the output.

Add a second patch generation via the diff function to make sure both
outputs are in sync.

Fixed GIT_DELTA_CONFLICTED not returned in some cases

racy-git, the missing link

If an index entry for a file that is not in HEAD is in conflicted state,
when diffing HEAD with the index, the status field of the corresponding git_diff_delta was incorrectly reported as GIT_DELTA_ADDED instead of GIT_DELTA_CONFLICTED.

This was due to handle_unmatched_new_item() initially setting the status
to GIT_DELTA_CONFLICTED but then overriding it later with GIT_DELTA_ADDED.

This fixes a bug where if a file was in conflicted state in either diff,
it would not always remain in conflicted state in the merged diff.

They fit there much better, even though we often check by diffing, it's
about the behaviour of the index.

When a file on the workdir has the same or a newer timestamp than the
index, we need to perform a full check of the contents, as the update of
the file may have happened just after we wrote the index.

The iterator changes are such that we can reach inside the workdir
iterator from the diff, though it may be better to have an accessor
instead of moving these structs into the header.

Zero out racily-clean entries' file_size