push: unset PARSE_OPT_OPTARG for --recurse-submodules

push: unset PARSE_OPT_OPTARG for --recurse-submodules

When the usage for `git push` is shown, it includes the following
lines

	--recurse-submodules[=(check|on-demand|no)]
			      control recursive pushing of submodules

which seem to indicate that the argument for --recurse-submodules is
optional. However, we cannot actually run that optiion without an
argument:

	$ git push --recurse-submodules
	fatal: recurse-submodules missing parameter

Unset PARSE_OPT_OPTARG so that it is clear that this option requires an
argument. Since the parse-options machinery guarantees that an argument
is present now, assume that `arg` is set in the else of
option_parse_recurse_submodules().

Reported-by: Andrew White <andrew.white@audinate.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>