tests: add tests of pathcopies() I'm working on support for storing copy metadata in the changeset instead of in the filelog. When storing it in the changeset, it will obviously be efficient to get the copy metadata for all files in a single changeset, but it will be more expensive to get the copy metadata all revisions of a single file. Some algorithms will then need to be optimized differently. The first method I'm going to rewrite is pathcopies(). This commit adds many tests for pathcopies(), so we can run the tests with both old and new versions of the code, as well as with metadata stored in filelog or in changeset (later). They use the debugpathcopies command I recently added (with no tests when it was added). They show a few bugs and few cases of slightly weird behavior. I'll fix the bugs in the next few commits. Differential Revision: https://phab.mercurial-scm.org/D5986