Fetch branches (or CI_REPO_DEFAULT_BRANCH)

[johanvdw]

I previously had a small drone plugin (code here) to compare what would be the size difference between a new commit and the default branch.
To enable that calculation, the plugin would also fetch the original branch and then compare sizes.
When moving to woodpecker, this no longer works because netrc is not injected in normal steps anymore.
I think it would be nice to be able to also fetch the default branch, or all branches similar to how tags can be fetched.

I can probably write a PR for this, but I'd first like to confirm I didn't miss an easy option to do the same thing already.

[qwerty287]

In v3 it will be possible to define plugins per repo that get the netrc creds as well.