Fix error with submodule cloning

Call `git submodule update --init` manually rather than using GitPython's
submodule API, as that fails with the following repo as a submodule:
https://files.creativekara.fr/git/naturalslopeslib.git

app/utils/git.py

@@ -60,9 +60,7 @@ def clone_repo(urlstr, ref=None, recursive=False):
 			origin.fetch()
 			repo.git.checkout(ref)
 
-			if recursive:
-				for submodule in repo.submodules:
-					submodule.update(init=True)
+			repo.git.submodule('update', '--init')
 
 		yield repo
 		shutil.rmtree(gitDir)