dim: Handle drm.git move to Gitlab automatically

As part of the transition of DRM from cgit to Gitlab, we updated
drm-rerere's nightly.conf file to reflect the new URL.

However, dim updates drm first and drm-tip lasts, so if you update the
git remote URL like we instructed before running dim update-branches (or
any command that fetch all remotes), it will error out before it has the
chance to retrieve the nightly.conf file.

If the user doesn't update the git remote URL, then dim update-branch
will run, fetch drm through the cgit repo, update nightly.conf. On the
second run, it will detect that the URL doesn't match anymore, but since
there's already a drm remote setup, it will fail to add the remote.

So, in either case, dim's broken.

Add a bit of logic that detects the drm remote URL and will update
remote url if we're using our legacy URLs.

Co-developed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <mripard@kernel.org>