Fix successful return value for SyncAndGetUserSpecificDiff (#27152) (#27156)

Backport #27152 by @delvh

A function should not return an error when it is successful.
Otherwise, things like
https://discord.com/channels/322538954119184384/322538954119184384/1153705341620600833
happen…

Co-authored-by: delvh <dev.lh@web.de>

docs/content/usage/actions/faq.en-us.md

@@ -180,3 +180,6 @@ For events supported only by GitHub, see GitHub's [documentation](https://docs.g
 | pull_request_review_comment | `created`, `edited`                                                                                                      |
 | release                     | `published`, `edited`                                                                                                    |
 | registry_package            | `published`                                                                                                              |
+
+> For `pull_request` events, in [GitHub Actions](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request), the `ref` is `refs/pull/:prNumber/merge`, which is a reference to the merge commit preview. However, Gitea has no such reference.
+> Therefore, the `ref` in Gitea Actions is `refs/pull/:prNumber/head`, which points to the head of pull request rather than the preview of the merge commit.

docs/content/usage/actions/faq.zh-cn.md

@@ -180,3 +180,6 @@ defaults:
 | pull_request_review_comment | `created`, `edited`                                                                                                      |
 | release                     | `published`, `edited`                                                                                                    |
 | registry_package            | `published`                                                                                                              |
+
+> 对于 `pull_request` 事件，在 [GitHub Actions](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request) 中 `ref` 是 `refs/pull/:prNumber/merge`，它指向这个拉取请求合并提交的一个预览。但是 Gitea 没有这种 reference。
+> 因此，Gitea Actions 中 `ref` 是 `refs/pull/:prNumber/head`，它指向这个拉取请求的头分支而不是合并提交的预览。

services/gitdiff/gitdiff.go

@@ -1312,7 +1312,7 @@ outer:
 		}
 	}
 
-	return diff, err
+	return diff, nil
 }
 
 // CommentAsDiff returns c.Patch as *Diff