tests: add test demonstrating buggy path handling
`hg debugupgraderepo` is currently buggy with regards to path
handling when copying files in .hg/store/. Specifically, it applies
the store filename encoding to paths instead of operating on raw
files.
This commit adds a test demonstrating the buggy behavior.
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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).