Rebase support

[gavv]

Currently this action uploads files to a repo by adding a new commit.

This is not always needed, because often we don't need to keep intermediate versions of artifacts from the same PR in history; we always need only the last one.

To achieve this, we can add an option ti support uploading via rebase + force push:

• clone repo
• remove all our older commits that upload the same artifact
• add a new commit on top that adds the artifact
• force-push to the repo

To avoid race conditions with other instances of the action that might also try to force-push, we should use --force-with-lease option. See manual page for git-push for details.