dim: support multiple remotes for branches in push&pull

This first patch just handles push and pull-request. Since the code
now auto-transfroms ssh:// git urls to git:// urls we can remove
$drm_intel_git.

Still left to do is fixing up create-branch and remove-branch.
Everything else should work here now.

v2: Rebase&fixup fallout.

v3: Squash in follow-up patch to rectify pull-request more.

v4: Also make it work properly again for drm-intel-next (which is not
in $integration_configuration and hence a special case again).

v5: s/get_remote_for_branch/branch_to_remote/ to align with
url_to_remote.  Also move next to url_to_remote.

v6: Move branch_to_repo to the right patch.

v7: Split out branch_to_remote, needed in an earlier patch.

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