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
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
new changesets d326ae2d01ee (1 drafts)
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
new changesets d326ae2d01ee (1 drafts)
4 files updated, 0 files merged, 0 files removed, 0 files unresolved