Re: t0021-conversion: flaky filter process test

> [...]
> 
> Now, in our tests commands are executed in rapid succession, so it's
> highly probable that those files are racily clean, and, consequently,
> 'git checkout' has to look at the contents of the files, meaning that
> it has to run the clean filter process, and then all is well: that's
> why the test succeeds most of the time.  Occasionally, however, the
> commands are executed slow enough or just at the right moment that the
> files are really clean, and 'git checkout' won't bother starting the
> clean filter process, which then won't write a 'debug.log' file,
> resulting in the above failure.