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.
```

models/pull/automerge.go

@@ -23,6 +23,7 @@ type AutoMerge struct {
 	Message                string                `xorm:"LONGTEXT"`
 	DeleteBranchAfterMerge bool
 	CreatedUnix            timeutil.TimeStamp `xorm:"created"`
+	MergedCommitID         string             `xorm:"VARCHAR(64)"`
 }
 
 // TableName return database table name for xorm
@@ -86,15 +87,6 @@ func GetScheduledMergePullIDsSince(ctx context.Context, since timeutil.TimeStamp
 	return pullIDs, err
 }
 
-// DeleteScheduledAutoMerge delete a scheduled pull request
-func DeleteScheduledAutoMerge(ctx context.Context, pullID int64) error {
-	exist, scheduledPRM, err := GetScheduledMergeByPullID(ctx, pullID)
-	if err != nil {
-		return err
-	} else if !exist {
-		return db.ErrNotExist{Resource: "auto_merge", ID: pullID}
-	}
-
-	_, err = db.GetEngine(ctx).ID(scheduledPRM.ID).Delete(&AutoMerge{})
-	return err
+func DeleteScheduledAutoMerge(ctx context.Context, pullID int64) (int64, error) {
+	return db.GetEngine(ctx).Where("pull_id = ?", pullID).Delete(&AutoMerge{})
 }