share: properly copy cache files when cloning from a share
If a is shared to b and b cloned to c, the old code would look directly
under b/.hg for the cache directory and not use the cachevfs layer to
pick the members from a/.hg/cache. Adjust variable names and comments to
reflect that the function is used for more than just the branchmap
cache.
Differential Revision: https://phab.mercurial-scm.org/D9598
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