Prepend commit message to template content (#20429)

- When a repository has a pull request template, it will always override
the current content. With this PR it will prepend content to the
template content when appropriate. This is similar how GitHub(and GitLab
I presume) does it and it saves developers time to not go open their
commit and copy paste their will written commit message.

routers/web/repo/compare.go

@@ -786,6 +786,19 @@ func CompareDiff(ctx *context.Context) {
 	ctx.Data["IsDiffCompare"] = true
 	ctx.Data["RequireTribute"] = true
 	setTemplateIfExists(ctx, pullRequestTemplateKey, nil, pullRequestTemplateCandidates)
+
+	// If a template content is set, prepend the "content". In this case that's only
+	// applicable if you have one commit to compare and that commit has a message.
+	// In that case the commit message will be prepend to the template body.
+	if templateContent, ok := ctx.Data[pullRequestTemplateKey].(string); ok && templateContent != "" {
+		if content, ok := ctx.Data["content"].(string); ok && content != "" {
+			// Re-use the same key as that's priortized over the "content" key.
+			// Add two new lines between the content to ensure there's always at least
+			// one empty line between them.
+			ctx.Data[pullRequestTemplateKey] = content + "\n\n" + templateContent
+		}
+	}
+
 	ctx.Data["IsAttachmentEnabled"] = setting.Attachment.Enabled
 	upload.AddUploadContext(ctx, "comment")