arrow

ARROW-16602: [Dev] Use GitHub API to merge pull request (#13184)

We use local "git merge" to merge a pull request in
dev/merge_arrow_pr.py.

If we use "git merge" to merge a pull request, GitHub's Web UI shows
"Closed" mark not "Merged" mark in a pull request page. This sometimes
confuses new contributors. "Why was my pull request closed without
merging?" See
https://github.com/apache/arrow/pull/12004#issuecomment-1031619771 for
example.

If we use GitHub API
https://docs.github.com/en/rest/pulls/pulls#merge-a-pull-request to
merge a pull request, GitHub's Web UI shows "Merged" mark not "Closed"
mark. See https://github.com/apache/arrow/pull/13180 for example. I
used GitHub API to merge the pull request.

And we don't need to create a local branch on local repository to
merge a pull request. But we must specify ARROW_GITHUB_API_TOKEN to
run dev/merge_arrow_pr.py.

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>