When checking out a file, we determine whether the baseline (what we
expect to be in the working directory) actually matches the contents
of the working directory.  This is safe behavior to prevent us from
overwriting changes in the working directory.

We look at the index to optimize this test: if we know that the index
matches the working directory, then we can simply look at the index
data compared to the baseline.

We have historically compared the baseline to the index entry by oid.
However, we must also compare the mode of the two items to ensure that
they are identical.  Otherwise, we will refuse to update the working
directory for a mode change.