Rework branch names to align them with git origins
Summary:
This modification is two-fold:
- For releases with a single sdist, generate a branch named refs/tags/<version>
- For releases with multiple sdists, generate branches named refs/tags/<ver>/<filename>
Depends on D423
Test Plan: tests have been updated
Reviewers: ardumont
Differential Revision: https://forge.softwareheritage.org/D424