Fix crash if git is not available

[agateau-gg]

Description

This PR fixes the crash which happens when running on a machine where git is not installed.

Before:

$ ggshield --help
Traceback (most recent call last):
  File "/root/.local/share/virtualenvs/src-NVTF7jWz/bin/ggshield", line 5, in <module>
    from ggshield.cmd.main import main
  File "/src/ggshield/cmd/main.py", line 10, in <module>
    from ggshield.cmd.auth import auth_group
  File "/src/ggshield/cmd/auth/__init__.py", line 5, in <module>
    from ggshield.cmd.common_options import add_common_options
  File "/src/ggshield/cmd/common_options.py", line 18, in <module>
    from ggshield.core.config.user_config import UserConfig
  File "/src/ggshield/core/config/__init__.py", line 1, in <module>
    from .auth_config import AccountConfig, InstanceConfig
  File "/src/ggshield/core/config/auth_config.py", line 20, in <module>
    from ggshield.core.utils import datetime_from_isoformat
  File "/src/ggshield/core/utils.py", line 16, in <module>
    from .git_shell import get_git_root, is_git_dir
  File "/src/ggshield/core/git_shell.py", line 41, in <module>
    GIT_PATH = get_git_path(os.getcwd())
  File "/src/ggshield/core/git_shell.py", line 24, in get_git_path
    raise Exception("unable to find git executable in PATH/PATHEXT")
Exception: unable to find git executable in PATH/PATHEXT

After:

$ ggshield --help
Usage: ggshield [OPTIONS] COMMAND [ARGS]...

Options:
  -c, --config-path FILE          Set a custom config file. Ignores local and
                                  global config files.
  --check-for-updates / --no-check-for-updates
                                  Check for ggshield updates.
  --allow-self-signed             Ignore ssl verification.
  --debug                         Show debug information.
  -v, --verbose                   Verbose display mode.
  --version                       Show the version and exit.
  -h, --help                      Show this message and exit.

Commands:
  api-status  Show API status.
  auth        Commands to manage authentication.
  config      Commands to manage configuration.
  iac         Commands to work with infrastructure as code.
  install     Install a pre-commit or pre-push git hook (local or global).
  quota       Show quotas overview.
  secret      Commands to work with secrets.

Scanning files work:

$ ggshield secret scan path setup.py
Scanning Path... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 1 files scanned out of 1 0:00:00

When trying to run a command which requires git:

$ ggshield secret scan commit-range HEAD~2..
unable to find git executable in PATH/PATHEXT

$ apt install git
[...]

$ ggshield secret scan commit-range HEAD~2..
Scanning Commits... ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 100% 2 commits scanned out of 2 0:00:00

Issue

Fixes #329.