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

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>