The logic to request a diff fragment with a base FileDiff ID was never
actually implemented. The RB.DiffReviewable model generated a URL with
an invalid query string and, even if it were valid, the
DiffFragmentView did not know how to ask for a single FileDiff with a
base FileDiff.
The RB.DiffReviewable model now generates a correct URL and the
DiffFragmentView can parse this into a call into get_diff_files with
the appropriate parameters.
Additionally, the base FileDiff determination in get_diff_files was
incorrect: it returned an ancestor that was too old. This has been
corrected to return the least recent FileDiff from after the specified
base commit.
