The community-driven ClearCase implementation included diffs of

directory elements (basically doing a diff of the ls output).

Directory diffs aren't a thing anywhere else in Review Board, but it's

apparently a requirement from our partners at HCL, so we're going to

reimplement this behavior.
This change adds back directory diffs, but does so in a hopefully

forward-looking way. We're including the correct "directory" type in the

diffx metadata to indicate that a file entry is actually for a

directory, and we're adding a new key to the VersionVault metadata

section to say that the directory diff is a legacy "filenames" diff.

This will allow us to implement directory diffs in a more useful/correct

way later down the line without having painted ourselves into a corner

relying on only this behavior.
I've added unit tests not only for the new directory diffs, but also for

basic file diff generation (in both legacy and diffx mode). While doing

this, I discovered a bug where creating diffs for the legacy mode with

dynamic views would end up replacing filenames twice, resulting in

filenames that looked like vob/vob/file, instead of vob/file. I've

changed it to explicitly replace the old and new filenames only on the

first and second lines of the diff.

Posted changes including directory diffs (along with a corresponding

  Power Pack change) and saw the correct output.
Ran unit tests.