unamend: abort if commit was not created by `hg [un]amend` `hg unamend` can currently undo any kind of rewrite, as long as it has an obsmarker. However, that has quite unexpected results if you run it after e.g. `hg rebase` (expecting it to behave like a generic `hg undo` command), because it updates to the predecessor and leaves the old changes in the working copy. I think it's better to allow `hg unamend` only after `hg amend` (and after `hg unamend` because that's documented as being supported). Differential Revision: https://phab.mercurial-scm.org/D12390

test: use `wait-on-file` in `test-racy-mutations.t` The official utility scale its timeout with the run-tests.py one. So lets use it. Differential Revision: https://phab.mercurial-scm.org/D12382