This adds an option to refresh the stat cache while generating
status.  It also rips out the GIT_PERF stuff I had an makes use
of the trace API to keep statistics about what happens during diff.

When diff is scanning the working directory, if it finds a file
where it is not sure if the index entry matches the working dir,
it will recalculate the OID (which is pretty expensive).  This
adds a new flag to diff so that if the OID calculation finds that
the file actually has not changed (i.e. just the modified time was
altered or such), then it will refresh the stat cache in the index
so that future calls to diff will not have to check the oid again.

This reorganized the diff OID calculation to make it easier to
correctly update the stat cache during a diff once the flags to
do so are enabled.

This includes marking the path of a git_index_entry as const so
we can make a "fake" git_index_entry with a "const char *" path
and not get warnings.  I was a little surprised at how unobtrusive
this change was, but I think it's probably a good thing.

In the same vein as the previous commits in this series.

This was never really working right because we were checking the
wrong flag and not checking it in all the places that we need to
be checking it.  I finally got around to writing a test and adding
actual support for it.

This makes no functional change to diff but renames a couple of
the objects and splits the new git_patch (formerly git_diff_patch)
into a new header file.

Add API for getting at git_diff_patch->content_size