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.
There was a little bug where the submodule cache thought that the
index date was out of date even when it wasn't that was resulting
in some extra scans of index data even when not needed.
Mostly this commit adds a bunch of new tests including adding and
removing submodules in the index and in the HEAD and seeing if we
can automatically pick them up when refreshing.
`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored. This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.
