dim: Improve updating of for-linux-next pointers

The current approach has a bunch of problems:
- It relies on the local maintainer's latest upstream version tags, so
  can flip-flop if they don't match.
- It's not really precise around the merge window.

The new approach should work a bit better:
- Unconditionally push -fixes.
- For the for-linux-next branch either push dinf or dinq. If dinf is ahead
  of -fixes then push dinf, assuming that drm-next is frozen already and
  that we've accumulated fixup patches already. If -fixes is ahead of dinf
  then push dinq assuming that the merge window has closed already.

This means we need to refine our process a bit to make sure this works
correctly:
- Only push -fixes forward to the new release past -rc1. Use dinf for fixup
  patches while the merge window is still open.
- As soon as -fixes has overtaken dinf don't roll it forward until drm-next
  is frozen for the next merge window.

v2: Run this in -nightly instead of -rerere