dim: Add apply and push shorthands for current branch

The new apply and push commands are shorthands for applying patches to
and pushing the current branch, respectively.

v2: use git symbolic-ref to find current branch (Daniel Vetter)

Signed-off-by: Thierry Reding <treding@nvidia.com>
[danvet: Fixup dim.rst as per Jani's review.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>