When formatting a patch as email we do not include the commit's
message in the formatted patch output. Implement this and add a
test that verifies behavior.

Now that non-pathspec matching diffs are implemented at the iterator
level, drop `FILELIST_MATCH`ing.

Document that `GIT_DIFF_PATHSPEC_DISABLE` is not necessarily about
explicit path matching, but also includes matching of directory
names.  Enforce this in a test.

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.