Provide more detailed messages when conditions pass or fail
unexpectedly.  In particular, this provides the error messages when a
test fails with a different error code than was expected.

Test that `git_submodule_update` can handle a submodule that is
freshly cloned and has a path differing from its name.

Test an initial submodule update, where we are trying to checkout
the submodule for the first time, and placing a file within the
submodule working directory with the same name as the submodule
(and consequently, the same name as the repository itself).

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

This lets us specify in the status call which ignore rules we want to
use (optionally falling back to whatever the submodule has in its
configuration).

This removes one of the reasons for having `_set_ignore()` set the value
in-memory. We re-use the `IGNORE_RESET` value for this as it is no
longer relevant but has a similar purpose to `IGNORE_FALLBACK`.

Similarly, we remove `IGNORE_DEFAULT` which does not have use outside of
initializers and move that to fall back to the configuration as well.

As submodules are becomes more like values, we should not let a status
check to update its properties. Instead of taking a submodule, have
status take a repo and submodule name.

Submodule init should handle relative paths in .gitmodules files
and resolve these urls when updating the git config file.