copies: add config option for writing copy metadata to file and/or changset This introduces a config option that lets you choose to write copy metadata to the changeset extras instead of to filelog. There's also an option to write it to both places. I imagine that may possibly be useful when transitioning an existing repo. The copy metadata is stored as two fields in extras: one for copies since p1 and one for copies since p2. I may need to add more information later in order to make copy tracing faster. Specifically, I'm thinking out recording which files were added or removed so that copies._chaincopies() doesn't have to look at the manifest for that. But that would just be an optimization and that can be added once we know if it's necessary. I have also considered saving space by using replacing the destination file path by an index into the "files" list, but that can also be changed later (but before the feature is ready to release). Differential Revision: https://phab.mercurial-scm.org/D6183