copies-tests: update to null after setup for output stability
This avoid having '@' moving around in graph output when the graph evolves.
after this change, graph evolution is still noisy, but less.
Differential Revision: https://phab.mercurial-scm.org/D8236
--- a/tests/test-copies-chain-merge.t Tue Mar 03 08:55:27 2020 +0100
+++ b/tests/test-copies-chain-merge.t Sat Feb 29 19:31:33 2020 +0100
@@ -299,8 +299,9 @@
final summary
+ $ hg update null --quiet
$ hg log -G
- @ 20 mEAm-0 simple merge - the other way]
+ o 20 mEAm-0 simple merge - the other way]
|\
+---o 19 mAEm-0 simple merge - one way]
| |/