parse-options: add a separate case for help output on error

parse-options: add a separate case for help output on error

When we parse a command line option such as -h or --help, we currently
exit 129, since that is the exit code when help output is printed. In a
future commit, we'll change this to exit 0 instead, since we're doing
what the user wanted successfully.

However, there are some cases where we print help output because the
user has provided ambiguous or invalid input, such as an ambiguous
option, and we'll want to exit unsuccessfully there.  Make this easier
by defining a new return code, PARSE_OPT_HELP_ERROR, that can be used in
this case, while reserving PARSE_OPT_HELP for those cases where the user
has requested help directly.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>