git rev-parse --parseopt tests: add more usagestr tests

git rev-parse --parseopt tests: add more usagestr tests

Add tests for the "usagestr" passed to parse-options.c
usage_with_options_internal() through cmd_parseopt().

These test for edge cases in the existing behavior related to the
"--parseopt" interface doing its own line-splitting with
strbuf_getline(), and the native C interface expecting and potentially
needing to handle newlines within the strings in the array it
accepts. The results are probably something that wasn't anticipated,
but let's make sure we stay backwards compatible with it.

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