dim: switch to using remote agnostic integration branch config

NOTE: This change depends on the nightly.conf commit 0d0e6d4e7dcc
("nightly.conf: prepare for remote agnostic configuration") in the
rerere-cache branch of the drm-intel repo. Looking at that first makes
this change more sensible.

Use two arrays to configure the repos and branches to be merged to the
integration branch:

drm_tip_repos

	An associative array that maps repo names to urls. This is
	mostly a convenience for defining the other array. The repo
	names are symbolic, and not related to actual git remote names.
	It's also helpful for implementing dim create-branch and
	remove-branch.

drm_tip_config

	An array of strings which describes the repos and branches to be
	used to generate the integration branch. The repos are listed
	using the symbolic repo names from the drm_tip_repos array. It's
	also possible to list an override sha, in case there's a need to
	hold back updating to the tip of the branch for some reason.

dim as a whole still remains dependent on specific (and configured)
remote names, but this change detaches nightly.conf from the remote
names.

Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>