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

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: wxiaoguang <wxiaoguang@gmail.com>
(cherry picked from commit ebf0c969403d91ed80745ff5bd7dfbdb08174fc7)

Conflicts:
	modules/private/hook.go
	routers/private/hook_post_receive.go
	trivial conflicts because
	  263a716cb5 * Performance optimization for git push (#30104)
	was not cherry-picked and because of
	  998a431747 Do not update PRs based on events that happened before they existed
(cherry picked from commit eb792d9f8a)

(cherry picked from commit ec3f5f9992d7ff8250c044a4467524d53bd50210)

modules/private/hook.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/modules/git"
+	"code.gitea.io/gitea/modules/repository"
 	"code.gitea.io/gitea/modules/setting"
 )
 
@@ -53,6 +54,7 @@ type HookOptions struct {
 	GitQuarantinePath               string
 	GitPushOptions                  GitPushOptions
 	PullRequestID                   int64
+	PushTrigger                     repository.PushTrigger
 	DeployKeyID                     int64 // if the pusher is a DeployKey, then UserID is the repo's org user.
 	IsWiki                          bool
 	ActionPerm                      int