`run`: Move libvcs to new `SubprocessCommand`, document + test overriding

[tony]

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requires #337

• run() should convert to Popen params for compatibility with SubprocessCommand

• run() should support legacy_params (existing) to deprecate safely

class CommandPopenMixin:
    _run_fn = Subprocess.Popen


class CommandRunMixin:
    _run_fn = Subprocess.run

class Command:
    _run_fn = SubprocessCommand




class Command:
    _run_fn = SubprocessCommand

This turns git command into a subprocess command factory:

class Git(Command):
   ...


git_factory = Git(url='...')
status_cmd = git_factory.status()
status_cmd.run()
status_cmd.Popen()

Subprocess.Popen
Subprocess.run