t/t4131-apply-fake-ancestor.sh: fix broken test

The third test "apply --build-fake-ancestor in a subdirectory" has been broken since it was introduced. It intended to modify a tracked file named 'sub/3.t' and then produce a diff which could be git apply'ed, but the file named 'sub/3.t' does not exist. The file that exists in the repo is called 'sub/3'. Since no tracked files were modified, an empty diff was produced, and the test succeeded. Correct this test by supplying the intended name of the tracked file, 'sub/3.t', to test_commit in the first test. Signed-off-by: Brandon Casey <drafnel@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>