Based on the ssh_config man pages relative includes should use
the default ssh config home (~/.ssh) as a base

Just as openssh, it will currently silently ignore missing files.

This solution is kind of hacky, as it breaks up the context blocks into
parts based on the `Include`, so that they are used in the order of
definition and not at the end of the block.

A proper solution would be to use an ordered collection of options,
during parsing, that ensures ordered lookup during the lookup phase.

OrderedDict was tried as a drop-in, but it is not sufficient enough,
as defined in the include-order-2 test case.

The storage of the parsed configs might need to be reviewed,
as the include support introduced complexity.

Due to how includes work, the order of the include is also relevant
during lookup time.

One possible solution would be to flatten the parsed config:
to compile the host/match conditions through includes during parsing.
This way the lookup could be much simpler.