t0021: make sure clean filter runs

In t0021.15 one of the things we are checking is that the clean filter
is run when checking out empty-branch.  The clean filter needs to be
run to make sure there are no modifications on the file system for the
test.r file, and thus it isn't dangerous to overwrite it.

However in the current test setup it is not always necessary to run
the clean filter, and thus the test sometimes fails, as debug.log
isn't written.

This happens when test.r has an older mtime than the index itself.
That mtime is also recorded as stat data for test.r in the index, and
based on the heuristic we're using for index entries, git correctly
assumes this file is up-to-date.

Usually this test succeeds because the mtime of test.r is the same as
the mtime of the index.  In this case test.r is racily clean, so git
actually checks the contents, for which the clean filter is run.

Fix the test by updating the mtime of test.r, so git is forced to
check the contents of the file, and the clean filter is run as the
test expects.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0021-conversion.sh

@@ -390,6 +390,9 @@ test_expect_success PERL 'required process filter should filter data' '
 		EOF
 		test_cmp_exclude_clean expected.log debug.log &&
 
+		# Make sure that the file appears dirty, so checkout below has to
+		# run the configured filter.
+		touch test.r &&
 		filter_git checkout --quiet --no-progress empty-branch &&
 		cat >expected.log <<-EOF &&
 			START