copies: move `merged` testing sooner

previously `is_merged` was an expensive callback. Now that all this data is
pre-computed, this is is a simple membership testing.

So it is probably cheaper than the membership testing.

Differential Revision: https://phab.mercurial-scm.org/D9141

mercurial/copies.py

@@ -397,9 +397,9 b' def _merge_copies_dict(minor, major, isa'
                 and dest in changes.salvaged
             ):
                 minor[dest] = value
-            elif not isancestor(new_tt, other_tt):
+            elif changes is not None and dest in changes.merged:
                 minor[dest] = value
-            elif changes is not None and dest in changes.merged:
+            elif not isancestor(new_tt, other_tt):
                 minor[dest] = value