copies: move `merged` testing sooner previously `is_merged` was an expensive callback. Now that all this data is pre-computed, this is is a simple membership testing. So it is probably cheaper than the membership testing. Differential Revision: https://phab.mercurial-scm.org/D9141

copies: return None instead of ChangingFiles when relevant If they are not relevant data, they are no need to fetch them. Differential Revision: https://phab.mercurial-scm.org/D9140

copies: add a HASCOPIESINFO flag to highlight rev with useful data If some files changes that may impact copy tracing are detected, we set this flag. This helps the copy tracing algorithm to skip fetching possibly expensive data when unnecessary. Differential Revision: https://phab.mercurial-scm.org/D9139

salvaged: properly deal with salvaged file during copy tracing When salvaged files are encountered, the removal have been reverted and we should keep the rename information from the other side. The conditional was starting to be quite hairy, so we spell it out in multiple elif case for readability. This fixes the associated test cases introduced a while back. The changeset centric copy tracing is now (known) bug free. Differential Revision: https://phab.mercurial-scm.org/D9130

salvaged: persist the salvaged set on disk With the new side data storage, this is trivial. Differential Revision: https://phab.mercurial-scm.org/D9129

changing-files: add clean computation of changed file for merges This is "a tad more complicated" than the previous cases. See inline documentation for details (have fun). Differential Revision: https://phab.mercurial-scm.org/D9128

changing-files: add clean computation of changed files for linear changesets The `files` field is not reliable, so we need to compute things from scratch. We deal with the second simplest case, linear changesets. We diff the current manifest with the parent manifest. This reveal the file added, changed and removed. Differential Revision: https://phab.mercurial-scm.org/D9127