mergecopies: invoke _computenonoverlap for both base and tca during merges The algorithm of _checkcopies can only walk backwards in the DAG, never forward. Because of this, the two _checkcopies patches need to run from their respective endpoints to the TCA to cover the entire subgraph where the merge is being performed. However, detection of files new in both endpoints, as well as directory rename detection, need to run with respect to the merge base, so we need lists of new files both from the TCA's and the merge base's viewpoint to correctly detect renames in a graft-like merge scenario. (Series reworked by Pierre-Yves David)

copies: make it possible to distinguish betwen _computenonoverlap invocations _computenonoverlap needs to be invoked twice during a graft, and debugging messages should be distinguishable between the two invocations

copies: make _checkcopies handle simple renames in a rotated DAG This introduces a distinction between "merge base" and "topological common ancestor". During a regular merge, these two are identical. Graft, however, performs a merge in a rotated DAG, where the merge base will not be a common ancestor at all in the original DAG. To correctly find copies in case of a graft, we need to take both the merge base and the topological CA into account, and track any renames between them in reverse. Fortunately we can detect this in advance, see comment in the code about "backwards". This patch only supports finding non-divergent renames contained entirely between the merge base and the topological CA. Further patches are coming to support more complex cases. (Pierre-Yves David was involved in the cleanup of this patch.)