dim: Make update_linux_next multi-repo compliant

- Only update the for-linux-next branches in the repo for the branch
  just pushed.
- Roll out the same logic (to allow us to always merge feature
  patches) for drm-misc.

v2: Bikesheds, bikesheds ... oh dear is this going to be fun to
re-test and then re-re-rebase out all the bugfixes in the right places
again.

v3: Fixup fallout - get_remote_for_branch wasn't working correctly.

v4: Don't use the nightly_ prefix, this is not about the integration
tree. Also sprinkle local annotations for the other read variables.

v5: s/map_nightly_remote/repo_to_remote/ to align with url_to_remote.

v6: s/get_nightly_repo_for_branch/branch_to_repo/ to align with
url_to_remote.  Also move next to url_to_remote.

v7: Split out branch_to_repo, already needed in an earlier patch.

v8: Really try to get rid of rebase fail, somehow a nightly_origin
got resurrected!

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