Add support for forking a single branch

This MR updates CreateFork() to support cloning all branches, or just --single-branch.

Related:

• Minimal forking: only clone default branch (gitlab#24894 - closed)
• (FE) Draft: Add possibility for minimal forks (gitlab!130616 - closed)
• (BE) Support forking all branches or just the defaul... (gitlab!130382 - closed)