Test that workdir diffs, when presented with UPDATE_INDEX, only
write the index when they actually make a change.

racy-git, the missing link

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.

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.

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.

Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.