test-lib functions: add --printf option to test_commit

test-lib functions: add --printf option to test_commit

Add a --printf option to test_commit to allow writing to the file with
"printf" instead of "echo".

This is useful for writing "\n", "\0" etc., in particular in
combination with the --append option added in 3373518cc8 (test-lib
functions: add an --append option to test_commit, 2021-01-12).

I'm converting a few tests to use the new option rather than a manual
printf/add/commit combination to demonstrate its usefulness. While I'm
at it use "test_create_repo" where appropriate, and give the
first/second commit a meaningful/more conventional log message in
cases where no test cared about that message.

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