Implement GIT_STATUS_OPT_EXCLUDE_SUBMODULES

Implement GIT_STATUS_OPT_EXCLUDE_SUBMODULES

This option has been sitting unimplemented for a while, so I
finally went through and implemented it along with some tests.

As part of this, I improved the implementation of
GIT_DIFF_IGNORE_SUBMODULES so it be more diligent about avoiding
extra work and about leaving off delta records for submodules to
the greatest extent possible (though it may include them still
if you are request TYPECHANGE records).