If a file is modified in the current diff and renamed / moved in the parent
diff, we have to use the file name from the parent diff as source file in
diffutils.get_original_file() otherwise the file diff will not be displayed
correctly.

This fixes bug 4573.

Testing done:

In a local Mercurial project, created a changeset that moves a file, and

subsequently a changeset the modifies the moved file.  Created a review for

the second changeset using rbt post.

Without the fix, the diff for the file is not displayed because the source

file cannot be found.

With the fix, the diff is displayed correctly.
A similar test has been performed by creating a copy of an existing file

in the first changeset.