merge with stable

copies: refactor checkcopies() into a top level method This moves checkcopies() out of mergecopies() and makes it a top level function in the copies module. This allows extensions to override it. For example, I'm developing a filelog replacement that doesn't have rev numbers so all the rev number dependent implementation here needs to be replaced by the extension. No logic is changed in this commit.

store: move top file walk to a separate function Some extensions find it useful to be able to walk the non-data files in the repo, so I'm moving that part of the walk to a separate function. In particular, this allows an extension to interact with only the non-filelog store data (for instance, when cloning everything but filelogs).