Generate proper diffs with moved file information for Perforce.

Modern versions of Perforce support file move operations. These may
either be straight moves, or moves with file changes. We now handle this
information and diff the files correctly. We also inject some custom
fields indicating the move information, so it can be processed by
Review Board.

This feature is funded by NetApp, Inc.

Added new unit tests, which test against the moved-files capability being
on and off. The tests were based on real-world usage of move operations in
Perforce.

Tested against a Perforce repository. Simulated moving files without changes,
and moving files with changes.

Tested the diff with moved file info against Review Board with support for these
new diff additions. That patch will go up separately.