parse-options: report uncorrupted multi-byte options

parse-options: report uncorrupted multi-byte options

Because our command-line parser considers only one byte at the time
for short-options, we incorrectly report only the first byte when
multi-byte input was provided. This makes user-errors slightly
awkward to diagnose for instance under UTF-8 locale and non-English
keyboard layouts.

Report the whole argument-string when a non-ASCII short-option is
detected.

Signed-off-by: Erik Faye-Lund <redacted>
Improved-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>