This introduces File.apply_patch(), which will apply patched file

content to the filesystem for full-repository tools. This takes care of

properly applying not just added/modified changes (which worked fine

before) but deleted, moved, and copied files (which didn't).
In the future, we may want to switch this to applying a patch through

GNU patch, to take advanage of time-tested patching logic. For now,

that's not how Review Bot works, so we're mimicking the logic we most

need for our tools.

New unit tests pass.