git-fastclone permits arbitrary shell command execution from .gitmodules

git-fastclone before 1.0.1 permits arbitrary shell command execution from .gitmodules. If an attacker can instruct a user to run a recursive clone from a repository they control, they can get a client to run an arbitrary shell command. Alternately, if an attacker can MITM an unencrypted git clone, they could exploit this. The ext command will be run if the repository is recursively cloned or if submodules are updated. This attack works when cloning both local and remote repositories.