wt-status: actually ignore submodules when requested

wt-status: actually ignore submodules when requested

Since ff6f1f564 (submodule-config: lazy-load a repository's .gitmodules
file, 2017-08-03) rebase interactive fails if there are any submodules
with unstaged changes which have been configured with a value for
'submodule.<name>.ignore' in the repository's config.

This is due to how configured values of 'submodule.<name>.ignore' are
handled in addition to a change in how the submodule config is loaded.
When the diff machinery hits a submodule (gitlink as well as a
corresponding entry in the submodule subsystem) it will read the value
of 'submodule.<name>.ignore' stored in the repository's config and if
the config is present it will clear the 'IGNORE_SUBMODULES' (which is
the flag explicitly requested by rebase interactive),
'IGNORE_UNTRACKED_IN_SUBMODULES', and 'IGNORE_DIRTY_SUBMODULES' diff
flags and then set one of them based on the configured value.

Historically this wasn't a problem because the submodule subsystem
wasn't initialized because the .gitmodules file wasn't explicitly loaded
by the rebase interactive command.  So when the diff machinery hit a
submodule it would skip over reading any configured values of
'submodule.<name>.ignore'.

In order to preserve the behavior of submodules being ignored by rebase
interactive, also set the 'OVERRIDE_SUBMODULE_CONFIG' diff flag when
submodules are requested to be ignored when checking for unstaged
changes.

Reported-by: Orgad Shaneh <orgads@gmail.com>
Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>