dim: Split out drm-tip.git

This is a prep step to eventually split out the drm-misc branches all
into their own repo. To get there we first need to split out the
integration tree mangling.

Since dim doesn't auto-update and since the transition is a good
reason to switch over to worktrees and it's tricky it's not scripted.
The following needs to be done when dim rebuild-nightly fails:

- Make sure you have the latest dim version.

- Good to also check you have a recent-ish git. Latest dim supports
  git worktree to save on disk space needed for .git metadata. It's
  much more robust than the git alternates storage trick we've used
  thus far. Note that git worktree is still optional (but that's not a
  recommended setup).

- Delete $DIM_PREFIX/{drm-intel-rerere,drm-intel-nightly,maintainer-tools}
  It's not strictly needed to kill the maintainer-tools checkout too,
  but might as well switch it over to git worktree too.

- Make sure you don't have any branches left over for drm-intel-rerere
  and drm-intel-nightly anywhere (since they need to switch their
  remotes).

- run dim setup and enjoy the new awesomeness.

If it breaks, I'm guilty ;-)

v2: don't call get_remote_name before switching directories.

v3: Rebase prep patches to be at the start of the series, which also
allows this patch to be squashed together with the one that renames
-nightly to -tip.

v4: Jumping over bikesheds.

v5: One more jump in read_integration_config.

v6: Use remote instead of origin in local variables, picked up from
Jani.

v7: Split out misplaced hunk.

v8: s/nightly_remote/remote, nightly_ should be only used for the
integration branch remotes. While at it also use grep -q instead of &>
/dev/null

v9: With nightly_remote free use that to denote the drm-tip remote in
rebuild-nightly.

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