rev-parse parseopt: do not search help text for flag chars

rev-parse parseopt: do not search help text for flag chars

When searching for flag characters in the option spec, we should ensure
the search stays within the bounds of the option spec and does not enter
the help text portion of the spec.  So when we find the boundary white
space marking the start of the help text, let's mark it with a nul
character.  Then when we search for flag characters starting from the
beginning of the string we'll stop at the nul and won't enter the help
text.

Now, the following option spec:

    exclame this does something!

will produce this 'set' expression when --exclame is specified:

    set -- --exclame --

instead of this one:

    set -- --exclame this does something --

Mark t1502.4 and t1502.5 as fixed.

Signed-off-by: Brandon Casey <redacted>
Signed-off-by: Junio C Hamano <redacted>