tests: demonstrate that copy info in changeset gets lost on amend

When copy information is stored in changesets, it gets lost on
amend. We didn't notice that until now because our users at Google
have the config set to `compatibility`, which means copy information
is stored in both changeset and filelogs.

Differential Revision: https://phab.mercurial-scm.org/D12386

tests/test-copies-in-changeset.t

@@ -316,6 +316,15 b' Test writing only to filelog'
   a -> k
 #endif
 
+Existing copy information is preserved by amend
+  $ hg cp a l
+  $ hg ci -m 'copy a to l'
+  $ hg showcopies
+  a -> l
+  $ hg ci --amend -m 'new description'
+  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/*-*-amend.hg (glob)
+  $ hg showcopies
+  a -> l (no-extra !)
   $ cd ..
 
 Test rebasing a commit with copy information