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.
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true