Starting in macOS Ventura, the diff command will be Apple Diff instead

of GNU Diff. This is based on BSD Diff with some GNU Diff

compatibility in the form of function arguments and general Unified Diff

output, but with minor differences in output.
This change implements the Apple Diff backend. It is very similar to the

GNU Diff backend, but without any instructions if missing (as Apple Diff

isn't something you'd install separately), and with some normalization

for output.
Normalization ensures that the timestamps are the same between Apple

Diff and GNU Diff. In GNU Diff, file timestamps have millisecond

precision and show the timezone offset. Apple Diff lacks both. This is

documented in the man page, and they state that running in legacy mode

(by setting COMMAND_MODE=legacy) will restore milliseconds and

timezone offsets. Rather than trust that legacy mode will exist

long-term, we handle normalization through our own process,

future-proofing to avoid problems if that behavior reverts back to GNU

Diff behavior.
They do note that millisecond precision and timezone offsets may cause

patches to fail to apply, but that doesn't appear to happen locally and

that would be a wider compatibility problem in the diff world anyway,

requiring normalization on the other end.
All clients that require an external diff tool have been updated to work

with Apple Diff. A notable client is TFS, which may generate a diff by

going directly through GNU Diff by way of rb-tfs, but it should still

be compatible.

All unit tests pass on a system with GNU Diff and on one with

Apple Diff.