implement a `CopyDirs` helper to walk all dirs and subdirs and copy all files within to a destination folder.
Refactor `TestRepairBadIndexVersion` to use the `CopyDirs` so it works on a copy db and remove the copy after the test.
This allows running the test more than once and doesn't confuse git.
Signed-off-by: Callum Stytan <callumstyan@gmail.com>