add local tree dry-run mode for rebuild-tip
Allow dim rebuild-tip to pull branches from local trees in dry-run mode. This is useful to confirm a backmerge is indeed correct, by comparing the resulting drm-tip with the old one. Currently the branch must be pushed out first for rebuild-tip to pick it up, which means the merge can't be fixed any more.
Also not just for merges, but in general to gauge how badly a patch to a branch would conflict the drm-tip rebuild.