context: introduce p[12]copies() methods and debugp[12]copies commands As mentioned earlier, I'm working on support for storing copy metadata in the changeset instead of in the filelog. In order to transition a repo from storing metadata in filelogs to storing it in the changeset, I'm going to provide a config option for reading the metadata from the changeset, but falling back to getting it from the filelog if it's not in the changeset. In this compatiblity mode, the changeset-optmized algorithms will be used. We will then need to convert the filelog copy metadata to look like that provided by changeset copy metadata. This patch introduces methods that do just that. By having these methods here, we can start writing changeset-optimized algorithms that should work already before we add any support for storing the metadata in the changesets. This commit also includes new debugp[12]copies commands and exercises them in test-copies.t. Differential Revision: https://phab.mercurial-scm.org/D5990