test-lib functions: add --author support to test_commit

test-lib functions: add --author support to test_commit

Add support for --author to "test_commit". This will simplify some
current and future tests, one of those is being changed here.

Let's also line-wrap the "git commit" command invocation to make diffs
that add subsequent options easier to add, as they'll only need to add
a new option line.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>