Galaxy_ng: shell injection in legacy role import via unsanitized git ref names

A command injection vulnerability was found in galaxy_ng. The do_git_checkout() function in the legacy role import API (v1) interpolates unsanitized git ref names (branch/tag names) into shell commands executed via subprocess.run() with shell=True. An authenticated user who controls a git repository can create a branch or tag with shell metacharacters in the name to achieve remote code execution on the pulp worker. The vulnerable endpoint is only reachable when GALAXY_ENABLE_LEGACY_ROLES is set to True, which is not the default configuration.