Perforce's patching support is limited, using the patch command line

tool with a bit of special handling for added and deleted files (which

has been busted for a long time).
The problems we had were that a capability-checking method for empty

files had been renamed at one point, and the regexes weren't broad

enough to handle anything but depot paths starting with //depot.
We now use the new Patcher API, implementing just the part that handles

added/deleted files, fixing them in the process.

Unit tests pass.