P4 Diff Mode is much more like Diff Mode.
Uses the same faces, the same font lock keywords, and some of the same navigation commands as Diff Mode.
The Diff Mode font lock keywords cover more diff syntaxes (not just unified).
Fixes #45.
Remove user option p4-colorized-diffs.
Fixes #43.
Use font locking in P4 Filelog Mode too.
Fix bug in p4-activate-diff-buffer.