t0012: test "-h" with builtins

t0012: test "-h" with builtins

Since commit 99caeed05 (Let 'git <command> -h' show usage
without a git dir, 2009-11-09), the git wrapper handles "-h"
specially, skipping any repository setup but still calling
the builtin's cmd_foo() function. This means that every
cmd_foo() must be ready to handle this case, but we don't
have any systematic tests. This led to "git am -h" being
broken for some time without anybody noticing.

This patch just tests that "git foo -h" works for every
builtin, where we see a 129 exit code (the normal code for
our usage() helper), and that the word "usage" appears in
the output.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>