cli: version: Fails if git is not installed

[pdxjohnny]

$ dffml version
Traceback (most recent call last):
  File "/usr/local/bin/dffml", line 8, in <module>
    sys.exit(CLI.main())
  File "/usr/local/lib/python3.9/dist-packages/dffml/util/cli/cmd.py", line 272, in main
    result = loop.run_until_complete(cls._main(*argv[1:]))
  File "/usr/lib/python3.9/asyncio/base_events.py", line 642, in run_until_complete
    return future.result()
  File "/usr/local/lib/python3.9/dist-packages/dffml/util/cli/cmd.py", line 238, in _main
    return await cls.cli(*args)
  File "/usr/local/lib/python3.9/dist-packages/dffml/util/cli/cmd.py", line 224, in cli
    return await cmd.do_run()
  File "/usr/local/lib/python3.9/dist-packages/dffml/util/cli/cmd.py", line 203, in do_run
    return await self.run()
  File "/usr/local/lib/python3.9/dist-packages/dffml/cli/cli.py", line 153, in run
    dirty, short_hash = await self.git_hash(path)
  File "/usr/local/lib/python3.9/dist-packages/dffml/cli/cli.py", line 99, in git_hash
    subprocess.call(
  File "/usr/lib/python3.9/subprocess.py", line 349, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/usr/lib/python3.9/subprocess.py", line 1823, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'git'
Error: Process completed with exit code 1.