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.

mercurial/branchmap.py

@@ -157,7 +157,9 b' class BranchMapCache:'
             for candidate in (b'base', b'immutable', b'served'):
                 rview = repo.filtered(candidate)
                 if cache.validfor(rview):
-                    cache = self._per_filter[candidate] = cache.copy(rview)
+                    cache._filtername = candidate
+                    self._per_filter[candidate] = cache
+                    cache._dirty = True
                     cache.write(rview)
                     return