copies: stop attempt to avoid extra dict copies around branching

In the python code, we attempt to avoid unnecessary dict copies when gathering
copy information. However that logic is wobbly and I keep running into case
where independent branches affects each others.

With the current code we can't ensure we are the only "user" of dict when
dealing with merge.

This caused havoc in the next series on tests I am about to introduce.

So for now I am disabling the faulty optimisation. I believe we will need a
dedicated overlay to deal with the "copy on write logic" to have something
correct. I am also hoping to find time to build dedicated test case for this
category of problem instead of relying on side effect in other tests. However
for now I am focussing on another issue.

Differential Revision: https://phab.mercurial-scm.org/D9608

mercurial/copies.py

@@ -383,9 +383,11 b' def _combine_changeset_copies('
 
                 if copies is None:
                     # this is a root
-                    copies = {}
-
-                newcopies = copies
+                    newcopies = copies = {}
+                elif remaining_children:
+                    newcopies = copies.copy()
+                else:
+                    newcopies = copies
                 # chain the data in the edge with the existing data
                 if changes is not None:
                     childcopies = {}
@@ -403,8 +405,6 b' def _combine_changeset_copies('
                             newcopies[dest] = (current_rev, source)
                         assert newcopies is not copies
                     if changes.removed:
-                        if newcopies is copies:
-                            newcopies = copies.copy()
                         for f in changes.removed:
                             if f in newcopies:
                                 if newcopies is copies:
@@ -417,9 +417,6 b' def _combine_changeset_copies('
                 # that child). See comment below for details.
                 if current_copies is None:
                     current_copies = newcopies
-                elif current_copies is newcopies:
-                    # nothing to merge:
-                    pass
                 else:
                     # we are the second parent to work on c, we need to merge our
                     # work with the other.