test-merge-tools: fix flaky test by avoiding debugsetparents debugsetparents is a debug command and does not provide the same guarantees as non-debug commands do. In particular, when the user sets a different parent, any clean files will remain clean in the dirstate even though the new parent might have a different version of the file (so it should appear modified compared to the new parent). Let's instead achieve the same effect by updating to the new parent and reverting the contents back to what they were. This fix can be tested by passing '--config debug.dirstate.delaywrite=2' to the 'hg update' command in the beforemerge().