test-parse-options: fix output when callback option fails

test-parse-options: fix output when callback option fails

When test-parse-options detects an error on the command line, it
gives the usage string just like any parse-options API users do,
without showing any "variable dump".  An exception is the callback
test, where a "variable dump" for the option is done before the
command line options are fully parsed.

Do not expose this implementation detail by separating the handling
of callback test into two phases, one to capture the fact that an
option was given during the option parsing phase, and the other to
show that fact as a part of normal "variable dump".

The effect of this fix is seen in the patch to t/t0040 where it
tried "test-parse-options --no-length" where "--length" is a callback
that does not take a negative form.

Signed-off-by: Junio C Hamano <gitster@pobox.com>