fix submodule lookup in bare repos

fix submodule lookup in bare repos

git_submodule_lookup does not work without a working tree [1], so the
current approach fails to recognize any submodules in bare repos.

Instead, notice that

	$ git ls-tree HEAD

lists any submodules as commit objects regardless of a working tree.
This is the only instance commit object is used in a tree, so we will
use this to check for submodules.

[1]: https://github.com/libgit2/libgit2/pull/4305/files