t7508: add tests capturing racy timestamp handling

"git status" fixes racy timestamps regardless of the worktree being
dirty or not. The new test cases capture this behavior.

Signed-off-by: Marc Strapetz <marc.strapetz@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7508-status.sh

@@ -1654,4 +1654,26 @@ test_expect_success '--no-optional-locks prevents index update' '
 	! test_is_magic_mtime .git/index
 '
 
+test_expect_success 'racy timestamps will be fixed for clean worktree' '
+	echo content >racy-dirty &&
+	echo content >racy-racy &&
+	git add racy* &&
+	git commit -m "racy test files" &&
+	# let status rewrite the index, if necessary; after that we expect
+	# no more index writes unless caused by racy timestamps; note that
+	# timestamps may already be racy now (depending on previous tests)
+	git status &&
+	test_set_magic_mtime .git/index &&
+	git status &&
+	! test_is_magic_mtime .git/index
+'
+
+test_expect_success 'racy timestamps will be fixed for dirty worktree' '
+	echo content2 >racy-dirty &&
+	git status &&
+	test_set_magic_mtime .git/index &&
+	git status &&
+	! test_is_magic_mtime .git/index
+'
+
 test_done