Test for commit concurrency errors

[carterm]

Migrated from...
cagov/wordpress-to-github#74

Had an auto update run while a human update happened. Got a commit patch error...
Error: 422 error making request patch https://api.github.com/repos/cagov/drought.ca.gov/git/refs/heads/main: "Unprocessable Entity"
Suspect it has something to do with moving the head, after someone else moved the head. Need to account for that.

With the use of github-tree-push, this is now only happening in commit-only mode. Would happen if a commit completed, but then during the compare before the commit could be moved, something changed on the branch. Could be testing by putting a breakpoint after the commit happens and then changing something in the branch somewhere else.

This is the API it uses.
https://docs.github.com/en/rest/reference/git#update-a-reference