I don't think a special script is necessary at first:
If those N
git repos are referenced together in a parent repo as submodules, you can go back in the history of that parent repo and get back all N
repos as they were versioned at the time.
Apply your git-bisect
on your parent repo, and make sure your test T
take advantage of the sources of the N
repos in N
sub-directories of that main repo.
That won't be as precise as a bisect done directly in the faulty submodule, but it can certainly help narrowing the search.
As in this blog post, your test T
might have to run a sub-git bisect
in each submodules.