test-lib functions: document arguments to test_commit

test-lib functions: document arguments to test_commit

The --notick argument was added in [1] and was followed by --signoff
in [2], but neither of these commits added any documentation for these
options. When -C was added in [3] a comment was added to document it,
but not the other options. Let's document all of these options.

1. 44b85e89d7 (t7003: add test to filter a branch with a commit at
   epoch, 2012-07-12),
2. 5ed75e2a3f (cherry-pick: don't forget -s on failure, 2012-09-14).
3. 6f94351b0a (test-lib-functions.sh: teach test_commit -C <dir>,
   2016-12-08)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>