Allow including `Reviewed-on`/`Reviewed-by` lines for custom merge messages (#31211)

This PR introduces the `ReviewedOn` and `ReviewedBy` variables for the
default merge message templates (e.g.,
`.gitea/default_merge_message/MERGE_TEMPLATE.md`).

This allows customizing the default merge messages while retaining these
trailers.

This also moves the associated logic out of `pull.tmpl` into the
relevant Go function.

This is a first contribution towards #11077.

---

For illustration, this allows to recreate the "default default" merge
message with the following template:
```
.gitea/default_merge_message/MERGE_TEMPLATE.md
Merge pull request '${PullRequestTitle}' (${PullRequestReference}) from ${HeadBranch} into ${BaseBranch}

${ReviewedOn}
${ReviewedBy}
```

3 files changed, 18 insertions, 8 deletions

diff --git a/docs/content/usage/merge-message-templates.en-us.md b/docs/content/usage/merge-message-templates.en-us.md
index fbdbd136f8..5116be3387 100644
--- a/docs/content/usage/merge-message-templates.en-us.md
+++ b/docs/content/usage/merge-message-templates.en-us.md
@@ -44,6 +44,8 @@ You can use the following variables enclosed in `${}` inside these templates whi
 - PullRequestIndex: Pull request's index number
 - PullRequestReference: Pull request's reference char with index number. i.e. #1, !2
 - ClosingIssues: return a string contains all issues which will be closed by this pull request i.e. `close #1, close #2`
+- ReviewedOn: Which pull request this commit belongs to. For example `Reviewed-on: https://gitea.com/foo/bar/pulls/1`
+- ReviewedBy: Who approved the pull request before the merge. For example `Reviewed-by: Jane Doe <jane.doe@example.com>`
 
 ## Rebase
 
diff --git a/services/pull/merge.go b/services/pull/merge.go
index 20be7c5b5a..6b5e9ea330 100644
--- a/services/pull/merge.go
+++ b/services/pull/merge.go
@@ -46,6 +46,9 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 	if err := pr.Issue.LoadPoster(ctx); err != nil {
 		return "", "", err
 	}
+	if err := pr.Issue.LoadRepo(ctx); err != nil {
+		return "", "", err
+	}
 
 	isExternalTracker := pr.BaseRepo.UnitEnabled(ctx, unit.TypeExternalTracker)
 	issueReference := "#"
@@ -53,6 +56,9 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 		issueReference = "!"
 	}
 
+	reviewedOn := fmt.Sprintf("Reviewed-on: %s/%s", setting.AppURL, pr.Issue.Link())
+	reviewedBy := pr.GetApprovers(ctx)
+
 	if mergeStyle != "" {
 		templateFilepath := fmt.Sprintf(".gitea/default_merge_message/%s_TEMPLATE.md", strings.ToUpper(string(mergeStyle)))
 		commit, err := baseGitRepo.GetBranchCommit(pr.BaseRepo.DefaultBranch)
@@ -77,6 +83,8 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 				"PullRequestPosterName":  pr.Issue.Poster.Name,
 				"PullRequestIndex":       strconv.FormatInt(pr.Index, 10),
 				"PullRequestReference":   fmt.Sprintf("%s%d", issueReference, pr.Index),
+				"ReviewedOn":             reviewedOn,
+				"ReviewedBy":             reviewedBy,
 			}
 			if pr.HeadRepo != nil {
 				vars["HeadRepoOwnerName"] = pr.HeadRepo.OwnerName
@@ -116,20 +124,22 @@ func getMergeMessage(ctx context.Context, baseGitRepo *git.Repository, pr *issue
 		return "", "", nil
 	}
 
+	body = fmt.Sprintf("%s\n%s", reviewedOn, reviewedBy)
+
 	// Squash merge has a different from other styles.
 	if mergeStyle == repo_model.MergeStyleSquash {
-		return fmt.Sprintf("%s (%s%d)", pr.Issue.Title, issueReference, pr.Issue.Index), "", nil
+		return fmt.Sprintf("%s (%s%d)", pr.Issue.Title, issueReference, pr.Issue.Index), body, nil
 	}
 
 	if pr.BaseRepoID == pr.HeadRepoID {
-		return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch), "", nil
+		return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch), body, nil
 	}
 
 	if pr.HeadRepo == nil {
-		return fmt.Sprintf("Merge pull request '%s' (%s%d) from <deleted>:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch), "", nil
+		return fmt.Sprintf("Merge pull request '%s' (%s%d) from <deleted>:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadBranch, pr.BaseBranch), body, nil
 	}
 
-	return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseBranch), "", nil
+	return fmt.Sprintf("Merge pull request '%s' (%s%d) from %s:%s into %s", pr.Issue.Title, issueReference, pr.Issue.Index, pr.HeadRepo.FullName(), pr.HeadBranch, pr.BaseBranch), body, nil
 }
 
 func expandDefaultMergeMessage(template string, vars map[string]string) (message, body string) {
diff --git a/templates/repo/issue/view_content/pull.tmpl b/templates/repo/issue/view_content/pull.tmpl
index 77378ef1bd..69e74da3a0 100644
--- a/templates/repo/issue/view_content/pull.tmpl
+++ b/templates/repo/issue/view_content/pull.tmpl
@@ -199,7 +199,6 @@
 
 				{{if .AllowMerge}} {{/* user is allowed to merge */}}
 					{{$prUnit := .Repository.MustGetUnit $.Context ctx.Consts.RepoUnitTypePullRequests}}
-					{{$approvers := (.Issue.PullRequest.GetApprovers ctx)}}
 					{{if or $prUnit.PullRequestsConfig.AllowMerge $prUnit.PullRequestsConfig.AllowRebase $prUnit.PullRequestsConfig.AllowRebaseMerge $prUnit.PullRequestsConfig.AllowSquash $prUnit.PullRequestsConfig.AllowFastForwardOnly}}
 						{{$hasPendingPullRequestMergeTip := ""}}
 						{{if .HasPendingPullRequestMerge}}
@@ -208,11 +207,10 @@
 						{{end}}
 						<div class="divider"></div>
 						<script type="module">
-							const issueUrl = window.location.origin + {{$.Issue.Link}};
 							const defaultMergeTitle = {{.DefaultMergeMessage}};
 							const defaultSquashMergeTitle = {{.DefaultSquashMergeMessage}};
-							const defaultMergeMessage = {{if .DefaultMergeBody}}{{.DefaultMergeBody}}{{else}}`Reviewed-on: ${issueUrl}\n` + {{$approvers}}{{end}};
-							const defaultSquashMergeMessage = {{if .DefaultSquashMergeBody}}{{.DefaultSquashMergeBody}}{{else}}`Reviewed-on: ${issueUrl}\n` + {{$approvers}}{{end}};
+							const defaultMergeMessage = {{.DefaultMergeBody}};
+							const defaultSquashMergeMessage = {{.DefaultSquashMergeBody}};
 							const mergeForm = {
 								'baseLink': {{.Link}},
 								'textCancel': {{ctx.Locale.Tr "cancel"}},