t1502: move optionspec help output to a file

t1502: move optionspec help output to a file

"git rev-parse --parseopt" shows the short help with its description of
all recognized options twice: When called with -h or --help, and after
reporting an unknown option.  Move the one for optionspec into a file
and use it in two tests to deduplicate that part.

"git rev-parse --parseopt -- --h" wraps the help text in "cat <<\EOF"
and "EOF".  Keep that part in the file to use it as is in the test that
needs it and simply remove it in the other one using sed.

Disable whitespace checking for the file using an attribute, as we need
to keep its spaces intact and wouldn't want a stray --whitespace=fix
turn them into tabs.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>