check-branches is vulnerable to command Injection

All versions of the package check-branches are vulnerable to Command Injection.

check-branches is a command-line tool that is interacted with locally, or via CI, to confirm no conflicts exist in git branches.

However, the library follows these conventions which can be abused:

1. It trusts branch names as they are (plain text)
2. It spawns git commands by concatenating user input

Since a branch name is potentially a user input - as users can create branches remotely via pull requests, or simply due to privileged access to a repository - it can effectively be abused to run any command.