Use multiprocessing to run diff in parallel.
Change-Id: I61e973d9c2cde039d5eebe8d0fe8bb63171ef447
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297483
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Chris Mcdonald <cjmcdonald@google.com>