Make it possible to set mode when uploading through API

[hramrach]

Feature Description

https://docs.gitea.com/api/1.24/#tag/repository/operation/repoChangeFiles does not document an option to set file mode.

When uploading the same files over API and over SSH the mode differs.

Screenshots

No response

[wxiaoguang]

Agree, I also found the legacy code is very hacky, need to fix the FIXME, then this issue can be resolved.