When a directory contains many submodules, we make a number of Gitaly
N+1 queries:

1. One TreeEntry RPC to look up each submodule
2. One TreeEntry RPC to read the `.gitmodules` file

The first item is redundant because we've already determined the tree
commit is a submodule. We can eliminate this query by skipping it
altogether.

The second item can be optimized by caching the contents of
`.gitmodules` once per directory. We already have a mechanism for
caching this in `SubmoduleLinks`. Now we just need to use it.

This significantly reduces Gitaly RPCs and closes two issues
with one stone:

1. https://gitlab.com/gitlab-org/gitlab/issues/35333
2. https://gitlab.com/gitlab-org/gitlab/issues/194380