gitstatus ignores the `ignore` field of .gitmodules

When used with a superproject that specifies submodules with the ignore field set to all, gitstatus will count it as an 'unstaged change' if the submodule is not at its own HEAD.

submodule configuration from .gitmodules:

[submodule "external/foo"]
        path = external/foo
        url = https://some.server.com/foo
        name = foo
        ignore = all

running git status doesn't show it as having unstaged changes:

╰─ git status
On branch bar
Your branch is ahead of 'origin/main' by 1 commit.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean

unless it's run with --ignore-submodules=<value other than 'all'>:

╰─ git status --ignore-submodules=dirty
On branch bar
Your branch is ahead of 'origin/main' by 1 commit.
  (use "git push" to publish your local commits)

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   external/foo (new commits)

no changes added to commit (use "git add" and/or "git commit -a")

but the prompt string constructed by gitstatus does count it as having unstaged changes:

bar:main ⇡1 !1 

Start by tracing gitstatus's submodule status handling and compare its result with Git's behavior when .gitmodules sets ignore = all. Reproduce the external/foo example and verify that the generated prompt no longer reports an unstaged change for the submodule.