Merge message template support for rebase without merge commit (#22669)

Use `default_merge_message/REBASE_TEMPLATE.md` for amending the message
of the last commit in the list of commits that was merged. Previously
this template was mentioned in the documentation but not actually used.

In this template additional variables `CommitTitle` and `CommitBody` are
available, for the title and body of the commit.

Ideally the message of every commit would be updated using the template,
but doing an interactive rebase or merging commits one by one is
complicated, so that is left as a future improvement.

1 files changed, 10 insertions, 2 deletions

diff --git a/services/pull/merge.go b/services/pull/merge.go
index 12e01e8ce7..85bb90b853 100644
--- a/services/pull/merge.go
+++ b/services/pull/merge.go
@@ -34,8 +34,8 @@ import (
 	issue_service "code.gitea.io/gitea/services/issue"
 )
 
-// GetDefaultMergeMessage returns default message used when merging pull request
-func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle) (message, body string, err error) {
+// getMergeMessage composes the message used when merging a pull request.
+func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle, extraVars map[string]string) (message, body string, err error) {
 	if err := pr.LoadBaseRepo(ctx); err != nil {
 		return "", "", err
 	}
@@ -81,6 +81,9 @@ func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr
 				vars["HeadRepoOwnerName"] = pr.HeadRepo.OwnerName
 				vars["HeadRepoName"] = pr.HeadRepo.Name
 			}
+			for extraKey, extraValue := range extraVars {
+				vars[extraKey] = extraValue
+			}
 			refs, err := pr.ResolveCrossReferences(ctx)
 			if err == nil {
 				closeIssueIndexes := make([]string, 0, len(refs))
@@ -133,6 +136,11 @@ func expandDefaultMergeMessage(template string, vars map[string]string) (message
 	return os.Expand(message, mapping), os.Expand(body, mapping)
 }
 
+// GetDefaultMergeMessage returns default message used when merging pull request
+func GetDefaultMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle) (message, body string, err error) {
+	return getMergeMessage(ctx, baseGitRepo, pr, mergeStyle, nil)
+}
+
 // Merge merges pull request to base repository.
 // Caller should check PR is ready to be merged (review and status checks)
 func Merge(ctx context.Context, pr *issues_model.PullRequest, doer *user_model.User, baseGitRepo *git.Repository, mergeStyle repo_model.MergeStyle, expectedHeadCommitID, message string, wasAutoMerged bool) error {