test-lib-functions.sh: fix usage for test_commit()

test-lib-functions.sh: fix usage for test_commit()

The usage comment for test_commit() shows that the --author option
should be given as `--author=<author>`. However, this is incorrect as it
only works when given as `--author <author>`. Correct this erroneous
text.

Also, for the sake of correctness, fix the description as well since we
invoke `git commit` with `--author <author>`, not `--author=<author>`.

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