Merge message template support for rebase without merge commit

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.

Author: brechtvl

services/pull/merge.go

@@ -34,8 +34,8 @@ import (
 	issue_service "code.gitea.io/gitea/services/issue"
 )
 
-// Compose message used when merging pull request.
-func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issues_model.PullRequest, mergeStyle repo_model.MergeStyle, commitTitle, commitBody string) (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
 	}
@@ -62,8 +62,6 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 		if err != nil {
 			if !git.IsErrNotExist(err) {
 				return "", "", err
-			} else if mergeStyle == repo_model.MergeStyleRebase {
-				return "", "", nil
 			}
 		} else {
 			vars := map[string]string{
@@ -79,14 +77,13 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 				"PullRequestIndex":       strconv.FormatInt(pr.Index, 10),
 				"PullRequestReference":   fmt.Sprintf("%s%d", issueReference, pr.Index),
 			}
-			if mergeStyle == repo_model.MergeStyleRebase {
-				vars["CommitTitle"] = commitTitle
-				vars["CommitBody"] = commitBody
-			}
 			if pr.HeadRepo != nil {
 				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))
@@ -141,7 +138,7 @@ func expandDefaultMergeMessage(template string, vars map[string]string) (message
 
 // 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, "", "")
+	return getMergeMessage(ctx, baseGitRepo, pr, mergeStyle, nil)
 }
 
 // Merge merges pull request to base repository.

services/pull/merge_rebase.go

@@ -12,20 +12,18 @@ import (
 	"code.gitea.io/gitea/modules/log"
 )
 
-// Compose message to amend commit in rebase merge of pull request.
+// getRebaseAmendMessage composes the message to amend commits in rebase merge of a pull request.
 func getRebaseAmendMessage(ctx *mergeContext, baseGitRepo *git.Repository) (message string, err error) {
 	// Get existing commit message.
-	var commitMessage strings.Builder
-	cmd := git.NewCommand(ctx, "show", "--format=%B", "-s")
-	if err := cmd.Run(&git.RunOpts{Dir: ctx.tmpBasePath, Stdout: &commitMessage}); err != nil {
+	commitMessage, _, err := git.NewCommand(ctx, "show", "--format=%B", "-s").RunStdString(&git.RunOpts{Dir: ctx.tmpBasePath})
+	if err != nil {
 		return "", err
 	}
 
-	commitTitle, commitBody, _ := strings.Cut(commitMessage.String(), "\n")
-	commitTitle = strings.TrimSpace(commitTitle)
-	commitBody = strings.TrimSpace(commitBody)
+	commitTitle, commitBody, _ := strings.Cut(commitMessage, "\n")
+	extraVars := map[string]string{"CommitTitle": strings.TrimSpace(commitTitle), "CommitBody": strings.TrimSpace(commitBody)}
 
-	message, body, err := getMergeMessage(ctx, baseGitRepo, ctx.pr, repo_model.MergeStyleRebase, commitTitle, commitBody)
+	message, body, err := getMergeMessage(ctx, baseGitRepo, ctx.pr, repo_model.MergeStyleRebase, extraVars)
 	if err != nil || message == "" {
 		return "", err
 	}
@@ -74,8 +72,7 @@ func doMergeRebaseFastForward(ctx *mergeContext) error {
 	}
 
 	if newMessage != "" {
-		cmdAmend := git.NewCommand(ctx, "commit", "--amend").AddOptionFormat("--message=%s", newMessage)
-		if err := cmdAmend.Run(&git.RunOpts{Dir: ctx.tmpBasePath}); err != nil {
+		if err := git.NewCommand(ctx, "commit", "--amend").AddOptionFormat("--message=%s", newMessage).Run(&git.RunOpts{Dir: ctx.tmpBasePath}); err != nil {
 			log.Error("Unable to amend commit message: %v", err)
 			return err
 		}