test: simplify `debugformat` matching in test-copies-chain-merge
This will make the test less sensible to unrelated format changing.
Differential Revision: https://phab.mercurial-scm.org/D11893
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls -A
.hg
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls -A
.hg
a