Move database operations of merging a pull request to post receive hook and add a...

Move database operations of merging a pull request to post receive hook and add a transaction (#30805) (#30888)

Backport #30805 by @lunny

Merging PR may fail because of various problems. The pull request may
have a dirty state because there is no transaction when merging a pull
request. ref
https://github.com/go-gitea/gitea/pull/25741#issuecomment-2074126393

This PR moves all database update operations to post-receive handler for
merging a pull request and having a database transaction. That means if
database operations fail, then the git merging will fail, the git client
will get a fail result.

There are already many tests for pull request merging, so we don't need
to add a new one.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>