dim: Simplify test for current branch

Instead of listing branches and grepping for the current branch, simply
read the name of the current branch and compare.

Suggested-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>