branchcache: stop using `copy(…)` in `replace(…)`
The `copy` method is mostly used for a filter level to inherit the branchmap
from a subset. So we stop using (abusing) it in "replace" to ensure `copy` is
used only for inheritance purposes.
Since `replace` is a method of the BranchMapCache, it seems fine to do lower
level operation there.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).