t3903-stash: Work around a racily-clean index that is not detected.

We still have the problem that sometimes the time returned by stat() is off
by a second into the past. As a consequence, git cannot detect when a file
was modified such that only the content changed but no other aspect of the
meta-data that stat() retrieves. So we artificially delay the modification
so that at least the timestamp is different from the one recorded in the
index.

t/t3903-stash.sh

@@ -92,6 +92,7 @@ test_expect_success 'drop middle stash' '
 	git reset --hard &&
 	echo 8 > file &&
 	git stash &&
+	sleep 1 &&
 	echo 9 > file &&
 	git stash &&
 	git stash drop stash@{1} &&