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