Error messages should be sentence fragments, and therefore:

1. Should not begin with a capital letter,
2. Should not conclude with punctuation, and
3. Should not end a sentence and begin a new one

Now that `git_diff_delta` data can be produced by reading patch
file data, which may have an abbreviated oid, allow consumers to
know that the id is abbreviated.

On platforms that lack `core.symlinks`, we should not go looking for
symbolic links and `p_readlink` their target.  Instead, we should
examine the file's contents.

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.

Introduce a new binary diff callback to provide the actual binary
delta contents to callers.  Create this data from the diff contents
(instead of directly from the ODB) to support binary diffs including
the workdir, not just things coming out of the ODB.

For consistency with the rest of the library, where an opt is an
options *structure*.