Our BaseSCMClient.merge and BaseSCMClient.create_commit methods

type the author argument as PatchAuthor. However, in our land

command, we actually pass a UserItemResource which we get from

review_request.get_submitter(). This was made evident in a bug where

rbt land would emit a warning about the author's profile being private

even though it wasn't. This happened because we try to access

author.full_name, which doesn't exist on the user resource.
This change addresses that bug and improves our handling of the case

where author information isn't available. Now the author argument

for these methods can be None, indicating that the author information

isn't available. All the SCM clients have been updated to handle this

accordingly.
The Git and Mercurial clients actually already had a unit test for this

case, and one has been added for Jujutsu.
This change also adds typing to the land command.

Ran unit tests.
Saw that the bug I was hitting with rbt land went away.

I don't have Jujutsu or Mercurial set up on my system so I

actually don't know if their unit tests for this pass. If possible

could someone apply this patch and confirm that the unit tests pass.