Use the original file name from the parent diff for files that are moved by the parent diff.

Review Request #9635 — Created Feb. 14, 2018 and discarded — Latest diff uploaded

Review Board

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

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.