Check only runs once and fails for new Submodules

[dennisconrad]

The check has two issues:

1. If a new submodule is added, commits will be empty and the check will fail with:

subprocess.CalledProcessError: Command '['git', 'ls-tree', '']' returned non-zero exit status 128.

2. The check will only run once, even if there are more than one changed submodule directory.

#2 fixes these issues.