test: simplify test-amend.t to avoid race condition
Insted on relying on sleep, we could simply have the editor do the file change.
This remove the reliance on "sleep" and avoid test failing on heavy load
machine.
To test this, I reverted the code change in
5558e3437872 and the test started
failing again.
This is a graft on stable of
141ceec06b55 which should have targeted for stable.
Differential Revision: https://phab.mercurial-scm.org/D8103
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