Currently, the `updateMirror` function which update the mirror interval and enable prune properties is only executed by the `Edit` function. But it is only triggered if `opts.MirrorInterval` is not null, even if `opts.EnablePrune` is not null.

With this patch, it is now possible to update the enable_prune property with a patch request without modifying the mirror_interval.

## Example request with httpie

### Currently:
**Does nothing**
```bash
http PATCH https://gitea.your-server/api/v1/repos/myOrg/myRepo "enable_prune:=false" -A bearer -a $gitea_token
```

**Updates both properties**
```bash
http PATCH https://gitea.your-server/api/v1/repos/myOrg/myRepo "enable_prune:=false" "mirror_interval=10m" -A bearer -a $gitea_token
```

### With the patch
**Updates enable_prune only**
```bash
http PATCH https://gitea.your-server/api/v1/repos/myOrg/myRepo "enable_prune:=false" -A bearer -a $gitea_token
```