tests: show invalid copies when turning off copies-in-changeset

If you turn on copies in changesets and write a commit with a copy,
then turn it off and amend the commit while undoing the copy, the
invalid copy information will remain. The read path doesn't crash in
invalid copy data, but it seems better to not produce the invalid
data.

Differential Revision: https://phab.mercurial-scm.org/D6751

tests/test-copies-in-changeset.t

@@ -133,6 +133,20 b' Test writing to both changeset and filel'
   a -> j
   $ hg showcopies --config experimental.copies.read-from=filelog-only
   a -> j
+Existing copy information in the changeset gets removed on amend and writing
+copy information on to the filelog
+  $ hg ci --amend -m 'copy a to j, v2' \
+  > --config experimental.copies.write-to=filelog-only
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/dd7bb9581359-a6e6b6d2-amend.hg
+  $ hg changesetcopies
+  files: j
+  filesadded: 0
+  filesremoved: 
+  
+  p1copies: 0\x00a (esc)
+  p2copies: 
+  $ hg showcopies --config experimental.copies.read-from=filelog-only
+  a -> j
 The entries should be written to extras even if they're empty (so the client
 won't have to fall back to reading from filelogs)
   $ echo x >> j