submodule foreach: fix recursion of options

submodule foreach: fix recursion of options

Calling

    git submodule foreach --recursive <subcommand> --<option>

leads to an error stating that the option --<option> is unknown to
submodule--helper. That is of course only, when <option> is not a valid
option for git submodule foreach.

The reason for this is, that above call is internally translated into a
call to submodule--helper:

    git submodule--helper foreach --recursive \
        -- <subcommand> --<option>

This call starts by executing the subcommand with its option inside the
first level submodule and continues by calling the next iteration of
the submodule foreach call

    git --super-prefix <submodulepath> submodule--helper \
      foreach --recursive <subcommand> --<option>

inside the first level submodule. Note that the double dash in front of
the subcommand is missing.

This problem starts to arise only recently, as the
PARSE_OPT_KEEP_UNKNOWN flag for the argument parsing of git submodule
foreach was removed in commit a282f5a906. Hence, the unknown option is
complained about now, as the argument parsing is not properly ended by
the double dash.

This commit fixes the problem by adding the double dash in front of the
subcommand during the recursion.

Signed-off-by: Morian Sonnet <moriansonnet@googlemail.com>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>