Assigning PRs to merger doesn't work when they're auto-merged

[jsoref]

A bunch of [@jsoref's] PRs are assigned to the people who merged them. It feels like the automation hasn't contemplated that PRs would be written by people who can't/won't be the people who will click the "merge" button.

This is intentional:

codeql-action/.github/update-release-branch.py

Lines 163 to 168 in bc64d12

	# Get the person who merged the pull request.
	# For most cases this will be the same as the author, but for PRs opened
	# by external contributors getting the merger will get us the GitHub
	# employee who reviewed and merged the PR.
	def get_merger_of_pr(repo, pr):
	return repo.get_commit(pr.merge_commit_sha).author.login

More interesting is that this approach seems to break when enabling auto-merge as can be seen here:
(merged by @angelapwen but PR is from @jsoref)

#2110 (@jsoref)

Originally posted by @intrigus-lgtm in #2131 (comment)