parse-options: show negatability of options in short help

Add a "[no-]" prefix to options without the flag PARSE_OPT_NONEG to
document the fact that you can negate them.

This looks a bit strange for options that already start with "no-", e.g.
for the option --no-name of git show-branch:

    --[no-]no-name        suppress naming strings

You can actually use --no-no-name as an alias of --name, so the short
help is not wrong.  If we strip off any of the "no-"s, we lose either
the ability to see if the remaining one belongs to the documented
variant or to see if it can be negated.

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

t/t1502-rev-parse-parseopt.sh

@@ -111,7 +111,7 @@ test_expect_success 'test --parseopt help-all output hidden switches' '
 |
 |    some-command does foo and bar!
 |
-|    --hidden1             A hidden switch
+|    --[no-]hidden1        A hidden switch
 |
 |EOF
 END_EXPECT