The old version relied on branch@{upstream}, which requires that
the branch is checked out. Instead use the indirection through the
abstract drm-tip repo.

v2: Questions from Jani:

- We still need the fallback path for non-managed branches like
  rerere-cache or drm-intel-next.

- Also this change removes some of the implicit validation that the
  branch has a local tracking branch. I only spotted one place where
  an assert_branch was missing.

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>