`extend_dirs` cann't process relative path

Hello,

I'm trying to use `extend_dirs` to publish a couple directories that live outside my hexo website (which is contained in a subdirectory of my main repo).

When I run `hexo deploy`, the directories I've identified are copied into the root of my website directory, but never get deployed. I think this is because only the `public` folder is being added to the `.deploy_git` directory. What is needed to actually deploy the contents of the `extend_dirs`?

I've included my file structure and deployment configuration below.

Thanks!

_EDIT: I tested this by creating a directory inside my website folder and referenced that in `extend_dirs`, and it deploys as expected. **It seems this issue only occurs when the directories exist outside the website folder**._

### File structure:
```
├─ repo
├─ docs
├─ src
├─ website
├─ public
├─ scaffolds
├─ source
├─ themes
├─ _config.yml
```

### _config.yml:
``` yaml
deploy:
type: git
repo:
branch: gh-pages
message:
extend_dirs:
- ../docs
- ../src
```