ggshield does not handle absence of git correctly

[agateau-gg]

GitGuardian Shield Version

• 1.13.0

Command executed

Start an Ubuntu 22.04 docker container: docker run -it --rm ubuntu:20.04

Install ggshield using pip:

apt update
apt install pip
pip install ggshield

Try to run ggshield:

# ggshield --help
Traceback (most recent call last):
  File "/usr/local/bin/ggshield", line 5, in <module>
    from ggshield.cmd.main import main
  File "/usr/local/lib/python3.10/dist-packages/ggshield/cmd/main.py", line 10, in <module>
    from ggshield.cmd.auth import auth_group
  File "/usr/local/lib/python3.10/dist-packages/ggshield/cmd/auth/__init__.py", line 3, in <module>
    from .login import login_cmd
  File "/usr/local/lib/python3.10/dist-packages/ggshield/cmd/auth/login.py", line 6, in <module>
    from ggshield.cmd.auth.utils import check_instance_has_enabled_flow
  File "/usr/local/lib/python3.10/dist-packages/ggshield/cmd/auth/utils.py", line 4, in <module>
    from ggshield.core.client import create_session
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/client.py", line 13, in <module>
    from .config import Config
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/config/__init__.py", line 2, in <module>
    from .config import Config
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/config/config.py", line 7, in <module>
    from ggshield.core.config.user_config import UserConfig
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/config/user_config.py", line 24, in <module>
    from ggshield.core.utils import api_to_dashboard_url
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/utils.py", line 14, in <module>
    from .git_shell import get_git_root, is_git_dir
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/git_shell.py", line 38, in <module>
    GIT_PATH = get_git_path(os.getcwd())
  File "/usr/local/lib/python3.10/dist-packages/ggshield/core/git_shell.py", line 21, in get_git_path
    raise Exception("unable to find git executable in PATH/PATHEXT")
Exception: unable to find git executable in PATH/PATHEXT

Describe the bug

ggshield assumes the git command is available at import time, and fails to start.

Expected behavior

At a minimum, ggshield should print a proper error message when git is not available, not a stack trace.

Ideally, ggshield should not require the git command unless it's necessary. Commands like secret scan docker or `secret scan archiveè do not require git to be available.

Tech details

Reworking git_shell.py to hide GIT_PATH and add a git() function should help.