fix: PR merge (#39442)

* Revert the behavior introduced by #30805
* Now the PR status is still managed in Gitea's code where the operation
is triggerred but not in post-receive hook
* Fix #39254 and many more related bugs.
    * Fix #39124

```
// MarkAsMerged sets a pull request to merged and closes the corresponding issue
// To make sure the pull request is marked as merged correctly, the caller uses multiple-stage operations:
//  1. Create a temp repo from base, merge the head into the temp repo, and get the merged commit ID and timestamp,
//  2. The merged commit ID and related information are stored into pull request
//  3. Push the merged commit to the base repo
//  4. Call MarkAsMerged to mark the pull request as merged and do post-processing (notification, close issues, etc)
//
// If failure occurs in step 1/2/3: the pull request is still open, the base repo is not changed, the doer can start a new merge.
// If failure occurs in step 4: the pull request can be marked as merged by the merged commit ID stored in it later.
```

cmd/hook.go

@@ -364,7 +364,6 @@ Gitea or set your environment appropriately.`, "")
 		GitPushOptions:                  pushOptions(),
 
 		PullRequestID: prID,
-		PushTrigger:   repo_module.PushTrigger(os.Getenv(repo_module.EnvPushTrigger)),
 
 		UserID:          pusherID,
 		UserName:        os.Getenv(repo_module.EnvPusherName),