t0021: fix flaky test

t0021: fix flaky test

t0021.15 creates files, adds them to the index, and commits them. All
this usually happens in a test run within the same second and Git cannot
know if the files have been changed between `add` and `commit`.  Thus,
Git has to run the clean filter in both operations. Sometimes these
invocations spread over two different seconds and Git can infer that the
files were not changed between `add` and `commit` based on their
modification timestamp. The test would fail as it expects the filter
invocation. Remove this expectation to make the test stable.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>