test: also deactivate `ui.merge` when testing merge tool
We are about to introduce a default config for `ui.merge`. Since
`test-merge-tools.t` is about testing merge configuration, we need to make sure
it won't interfere.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).