Previously, if File A (at orig) was renamed to File B (at revision 1),

and then if File B was subsequently renamed to File C (at revision 2),

then the diff viewer would correctly display the diff from orig to

revision 1 as A -> B and correctly display the diff from orig to

revision 2 as A -> C. However, the interdiff from revision 1 to

revision 2 would be incorrectly displayed as A -> B instead of the

expected B -> C.
This change addresses this issue by determining the depot_filename and

dest_filename of each diff_file based on whether or not its

corresponding interfilediff part exists.

Ran and passed the relevant reviewboard unit tests.
Added a unit test that covers this bug, which passes.
Manually verified that interdiff filenames displayed correctly, even

  with multiple renames.