parse-options: fix SunCC compiler warning

parse-options: fix SunCC compiler warning

The compiler reports this because show_gitcomp() never actually
returns a value:

    "parse-options.c", line 520: warning: Function has no return
    statement : show_gitcomp

We could shut the compiler up. But instead let's not bury exit() too
deep. Do the same as internal -h handling, return a special error code
and handle the exit() in parse_options() (and other
parse_options_step() callers) instead.

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>