Ensure that we cannot modify a file after it's been renamed out of the
way.  If multiple deltas exist for a single path, ensure that we do not
attempt to modify a file after it's been renamed out of the way.

To support this, we must track the paths that have been removed or
renamed; add to a string map when we remove a path and remove from the
string map if we recreate a path.  Validate that we are not applying to
a path that is in this map, unless the delta is a rename, since git
supports renaming one file to two different places in two different
deltas.

Further, test that we cannot apply a modification delta to a path that
will be created in the future by a rename (a path that does not yet
exist.)