copies: remove existing copy info from the changeset on amend (BC)
When amending a changeset with copy information in the changeset and
the new changeset doesn't have any copy information (or similar for
"filesadded" and "filesremoved"), we shouldn't keep it.
A drawback of this is that we now unconditionally remove these four
entries from the extras, breaking any extensions that happened to
write entries with the same names (which seems very unlikely).
I think I'd heard that there was list of blacklisted keys that would
be removed from the extras when a commit is rewritten, but I couldn't
find that. It would make sense to add the keys mentioned above there
instead of the custom filtering I've added in this patch.
Differential Revision: https://phab.mercurial-scm.org/D6752
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