dim: Adapt create/remove-branch

Looking up the remote for deleting the branch is a bit an exercise.

v2: Correctly keep the fake remote name from nightly.conf and the real
(local) remote name apart.

v3: More untested rebasing. This removes the FIXME comments and compat
code from Jani, since these two functions should work now properly.

v4: Now tested, and with fallout fixed - map_nightly_remote was
was missing, and also accidently left some old garbage in.

v5: s/map_nightly_remote/repo_to_remote/ to align with url_to_remote.
Also move next to url_to_remote.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>