Remote branches are not checked out (git)

Precondition: #293 is applied.

Expected: If one passes an pipurl with a ...@some-branch the branch should be picked.

Current behavior: It does not checkout the branch, but instead rebases the remote branch into master (or main). Which sometimes fails, dependent if there are conflicts or not. Anyway, the result is wrong.

Solution: Checkout the remote branch before rebase.
