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.
We update the index and then immediately change the contents of the
file. This makes the diff think there are no changes, as the timestamp
of the file agrees with the cached data. This is however a bug, as the
file has obviously changed contents.
The test is a bit fragile, as it assumes that the index writing and the
following modification of the file happen in the same second, but it's
enough to show the issue.
