Retarget depending pulls when the parent branch is deleted (#28686)

Sometimes you need to work on a feature which depends on another (unmerged) feature.
In this case, you may create a PR based on that feature instead of the main branch.
Currently, such PRs will be closed without the possibility to reopen in case the parent feature is merged and its branch is deleted.
Automatic target branch change make life a lot easier in such cases.
Github and Bitbucket behave in such way.

Example:
$PR_1$: main <- feature1
$PR_2$: feature1 <- feature2

Currently, merging $PR_1$ and deleting its branch leads to $PR_2$ being closed without the possibility to reopen.
This is both annoying and loses the review history when you open a new PR.

With this change, $PR_2$ will change its target branch to main ($PR_2$: main <- feature2) after $PR_1$ has been merged and its branch has been deleted.

This behavior is enabled by default but can be disabled.
For security reasons, this target branch change will not be executed when merging PRs targeting another repo. 

Fixes #27062
Fixes #18408

---------

Co-authored-by: Denys Konovalov <kontakt@denyskon.de>
Co-authored-by: delvh <dev.lh@web.de>

services/pull/pull.go

@@ -546,6 +546,43 @@ func (errs errlist) Error() string {
 	return ""
 }
 
+// RetargetChildrenOnMerge retarget children pull requests on merge if possible
+func RetargetChildrenOnMerge(ctx context.Context, doer *user_model.User, pr *issues_model.PullRequest) error {
+	if setting.Repository.PullRequest.RetargetChildrenOnMerge && pr.BaseRepoID == pr.HeadRepoID {
+		return RetargetBranchPulls(ctx, doer, pr.HeadRepoID, pr.HeadBranch, pr.BaseBranch)
+	}
+	return nil
+}
+
+// RetargetBranchPulls change target branch for all pull requests whose base branch is the branch
+// Both branch and targetBranch must be in the same repo (for security reasons)
+func RetargetBranchPulls(ctx context.Context, doer *user_model.User, repoID int64, branch, targetBranch string) error {
+	prs, err := issues_model.GetUnmergedPullRequestsByBaseInfo(ctx, repoID, branch)
+	if err != nil {
+		return err
+	}
+
+	if err := issues_model.PullRequestList(prs).LoadAttributes(ctx); err != nil {
+		return err
+	}
+
+	var errs errlist
+	for _, pr := range prs {
+		if err = pr.Issue.LoadRepo(ctx); err != nil {
+			errs = append(errs, err)
+		} else if err = ChangeTargetBranch(ctx, pr, doer, targetBranch); err != nil &&
+			!issues_model.IsErrIssueIsClosed(err) && !models.IsErrPullRequestHasMerged(err) &&
+			!issues_model.IsErrPullRequestAlreadyExists(err) {
+			errs = append(errs, err)
+		}
+	}
+
+	if len(errs) > 0 {
+		return errs
+	}
+	return nil
+}
+
 // CloseBranchPulls close all the pull requests who's head branch is the branch
 func CloseBranchPulls(ctx context.Context, doer *user_model.User, repoID int64, branch string) error {
 	prs, err := issues_model.GetUnmergedPullRequestsByHeadInfo(ctx, repoID, branch)