refactor(automerge): fix error handling, populate recent automerge tasks on restart (#39001)

* Refactor "automerge" related code, clarify many details (including "unique queue item", start check by pull head or commit)
* Fix automerge queue handler's error handling, clarify error messages
* Populate recent automerge tasks on restart to restore the previous aborted automerge tasks

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

services/pull/ref.go

@@ -0,0 +1,55 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package pull
+
+import (
+	"context"
+	"errors"
+
+	issues_model "gitea.dev/models/issues"
+	repo_model "gitea.dev/models/repo"
+	"gitea.dev/modules/git"
+	"gitea.dev/modules/util"
+)
+
+func EnumPullRequestsByHeadCommitID(ctx context.Context, repo *repo_model.Repository, commitID string, filter func(*issues_model.PullRequest) bool) (pulls []*issues_model.PullRequest, _ error) {
+	gitRepo, err := git.OpenRepository(ctx, repo)
+	if err != nil {
+		return nil, err
+	}
+	defer gitRepo.Close()
+
+	refs, err := gitRepo.GetRefsBySha(ctx, commitID, git.PullPrefix)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, refStr := range refs {
+		ref := git.RefName(refStr)
+		prIndex, ok := ref.PullIndex()
+		if !ok {
+			continue
+		}
+
+		pull, err := issues_model.GetPullRequestByIndex(ctx, repo.ID, prIndex)
+		if err != nil {
+			if errors.Is(err, util.ErrNotExist) {
+				continue // ignore non-existing pull requests
+			}
+			return nil, err
+		}
+
+		if filter(pull) {
+			pulls = append(pulls, pull)
+		}
+	}
+	return pulls, nil
+}
+
+func GetMergeablePullRequestsByHeadCommitID(ctx context.Context, repo *repo_model.Repository, commitID string) ([]*issues_model.PullRequest, error) {
+	return EnumPullRequestsByHeadCommitID(ctx, repo, commitID, func(pr *issues_model.PullRequest) bool {
+		_ = pr.LoadIssue(ctx)
+		return pr.Issue != nil && !pr.Issue.IsClosed && !pr.HasMerged && pr.IsStatusMergeable()
+	})
+}