t0021-conversion: flaky filter process test

Hi Lars,

I see occasional failures in the test 'required process filter should filter data' in 't0021-conversion.sh', added in edcc85814c (convert: add filter.<driver>.process option, 2016-10-16); most often in the macOS Travis CI build jobs, but it's an OS-independent timing issue.

That's an enormous test, and the part failing every once in a while is this:

    filter_git checkout --quiet --no-progress empty-branch &&
    cat >expected.log <<-EOF &&
            START
            init handshake complete
            IN: clean test.r $S [OK] -- OUT: $S . [OK]
            STOP
    EOF
    test_cmp_exclude_clean expected.log debug.log &&

Here's the relevant trace output:

  ++filter_git checkout --quiet --no-progress empty-branch
  ++rm -f debug.log expected.log
  ++git checkout --quiet --no-progress empty-branch
  ++cat
  ++test_cmp_exclude_clean expected.log debug.log
  ++expect=expected.log
  ++actual=debug.log
  ++for FILE in '"$expect"' '"$actual"'
  ++grep -v 'IN: clean' expected.log
  ++for FILE in '"$expect"' '"$actual"'
  ++grep -v 'IN: clean' debug.log
  grep: debug.log: No such file or directory
  error: last command exited with $?=2

So when checking out the 'empty-branch' four tracked files from the currently checked out 'master' branch must be removed, and this part of the test assumes that the filter process is always invoked to clean those files. Alas, this is not necessarily the case: if 'git checkout' knows for sure that all files are clean, then it won't waste resources running the clean filter process at all, and just deletes those files.

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.

You might find the 'sg/stress-test' topic (already in 'next', tip at fb7d1e3ac8; just run './t0021-conversion.sh -r 1,15 --stress' and wait) and the patch below helpful in reproducing and debugging the issue.