api: GetPullRequestCommits: return file list (#27483) (#27539)

api: GetPullRequestCommits: return file list (#27483) (#27539)

Backport #27483 by @msantos

Fixes https://github.com/go-gitea/gitea/issues/27481

---
Patch tested:

```json
[
  {
    "url": "http://100.115.92.198:9292/api/v1/repos/msantos/test/git/commits/7664dcb44167e0f9efd994e4ca6a9164694adc27",
    "sha": "7664dcb44167e0f9efd994e4ca6a9164694adc27",
    "created": "2023-10-06T09:57:08-04:00",
    "html_url": "http://100.115.92.198:9292/msantos/test/commit/7664dcb44167e0f9efd994e4ca6a9164694adc27",
...
    "files": [
      {
        "filename": "README.md",
        "status": "modified"
      }
    ],
    "stats": {
      "total": 2,
      "additions": 2,
      "deletions": 0
    }
  }
]
```

Co-authored-by: Michael Santos <michael.santos@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>