Speed up mirror-generation through parallelization

[Vinai]

@tdgroot had the idea to enable parallelized builds by making a copy of the git repo for each tag.
This would allow each tag to be processed in parallel, which would make a pretty big difference for the mirror-magento2 repo.

[Vinai]

A bit of complexity is added because for mage-os releases, the working copies are used to push the release tags back to GitHub.
So either that process will have to be adjusted to use the changed directory structure, or we limit the parallelization to the mirror generation where it is needed. The release build process would not benefit from it because it doesn't operate on multiple tags.