t7519: avoid file to index mtime race for untracked cache

In t7519 there is a test that writes files to disk, and immediately
writes the index with the untracked cache. Because of
mtime-comparison logic that uses a 1-second resolution, this means
the cached entries are not trusted/used under some circumstances
(see read-cache.c#is_racy_stat()).

Untracked cache tests in t7063 use a 1-second delay to avoid this
issue, but we don't want to introduce arbitrary slowdowns, so instead
use test-tool chmtime to backdate the files slightly. The t7063
delays are a #leftoverbit, to be worked on in a separate series.

This change doesn't actually affect the outcome of the test, but does
enhance its validity, and becomes relevant after later changes.

Signed-off-by: Tao Klerks <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: TaoK

t/t7519-status-fsmonitor.sh

@@ -324,13 +324,19 @@ test_expect_success UNTRACKED_CACHE 'ignore .git changes when invalidating UNTR'
 		cd dot-git &&
 		mkdir -p .git/hooks &&
 		: >tracked &&
+		test-tool chmtime =-60 tracked &&
 		: >modified &&
+		test-tool chmtime =-60 modified &&
 		mkdir dir1 &&
 		: >dir1/tracked &&
+		test-tool chmtime =-60 dir1/tracked &&
 		: >dir1/modified &&
+		test-tool chmtime =-60 dir1/modified &&
 		mkdir dir2 &&
 		: >dir2/tracked &&
+		test-tool chmtime =-60 dir2/tracked &&
 		: >dir2/modified &&
+		test-tool chmtime =-60 dir2/modified &&
 		write_integration_script &&
 		git config core.fsmonitor .git/hooks/fsmonitor-test &&
 		git update-index --untracked-cache &&