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.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}