tach test assumes the main branch is called main #888
Description
$ tach test
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/tach/filesystem/git_ops.py", line 163, in get_changed_files
INTERNALERROR> diff: str = repo.git.diff("--name-status", base)
INTERNALERROR> ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/git/cmd.py", line 1003, in <lambda>
INTERNALERROR> return lambda *args, **kwargs: self._call_process(name, *args, **kwargs)
INTERNALERROR> ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/git/cmd.py", line 1616, in _call_process
INTERNALERROR> return self.execute(call, **exec_kwargs)
INTERNALERROR> ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/git/cmd.py", line 1406, in execute
INTERNALERROR> raise GitCommandError(redacted_command, status, stderr_value, stdout_value)
INTERNALERROR> git.exc.GitCommandError: Cmd('git') failed due to: exit code(128)
INTERNALERROR> cmdline: git diff --name-status main
INTERNALERROR> stderr: 'fatal: ambiguous argument 'main': unknown revision or path not in the working tree.
INTERNALERROR> Use '--' to separate paths from revisions, like this:
INTERNALERROR> 'git <command> [<revision>...] -- [<file>...]''
INTERNALERROR>
INTERNALERROR> During handling of the above exception, another exception occurred:
INTERNALERROR>
INTERNALERROR> Traceback (most recent call last):
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/_pytest/main.py", line 314, in wrap_session
INTERNALERROR> config._do_configure()
INTERNALERROR> ~~~~~~~~~~~~~~~~~~~~^^
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/_pytest/config/__init__.py", line 1159, in _do_configure
INTERNALERROR> self.hook.pytest_configure.call_historic(kwargs=dict(config=self))
INTERNALERROR> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/pluggy/_hooks.py", line 534, in call_historic
INTERNALERROR> res = self._hookexec(self.name, self._hookimpls.copy(), kwargs, False)
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/pluggy/_manager.py", line 120, in _hookexec
INTERNALERROR> return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
INTERNALERROR> ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/pluggy/_callers.py", line 167, in _multicall
INTERNALERROR> raise exception
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/pluggy/_callers.py", line 121, in _multicall
INTERNALERROR> res = hook_impl.function(*args)
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/tach/pytest_plugin.py", line 51, in pytest_configure
INTERNALERROR> changed_files = get_changed_files(**kwargs)
INTERNALERROR> File "/home/me/pytest-robotframework/.venv/lib/python3.13/site-packages/tach/filesystem/git_ops.py", line 166, in get_changed_files
INTERNALERROR> raise TachError(f"Failed to check diff between '{base}' and {head_display}!")
INTERNALERROR> tach.errors.TachError: Failed to check diff between 'main' and current filesystem!
it could be called master or something else. pretty sure there's a way to reliably determine the default branch name, so we should do that instead of making this assumption