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).

This implements working versions of GIT_DIFF_RECURSE_IGNORED_DIRS
and GIT_STATUS_OPT_RECURSE_IGNORED_DIRS along with some tests for
the newly available behaviors.  This is not turned on by default
for status, but can be accessed via the options to the extended
version of the command.

This improves docs in some of the public header files, cleans
up and improves some of the example code, and fixes a couple
of pedantic warnings in places.

Support TYPECHANGE records in status and adjust checkout accordingly

To answer if a single given file should be ignored, the path to
that file has to be processed progressively checking that there
are no intermediate ignored directories in getting to the file
in question.  This enables that, fixing the broken old behavior,
and adds tests to exercise various ignore situations.