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.

Ran unit tests.