Add an option to `git replay` so it can do reverts

Now that the patch series creating the git replay command has been merged into the master branch, we can improve this new command and add useful features.

The command can do cherry-picks, but it cannot do reverts. They are quite similar to cherry-picks though, so they might not be difficult to implement and could help simplify our revert code in a similar way as the command could simplify our cherry-pick code.

It especially worth doing as we plan to use the command to simplify and speed up Gitaly, and this feature would probably help.

/cc @jcaigitlab