parse-options: allow -h as a short option

parse-options: allow -h as a short option

Let callers provide their own handler for the short option -h even
without the flag PARSE_OPT_NO_INTERNAL_HELP, but call the internal
handler (showing usage information) if that is the only parameter.
Implement the first part by checking for -h only if parse_short_opt()
can't find it and returns -2.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Jeff King <redacted>