Parse a commit command.

def _parse_commit(self, ref):
        """Parse a commit command."""
        lineno  = self.lineno
        mark = self._get_mark_if_any()
        author = self._get_user_info(b'commit', b'author', False)
        more_authors = []
        while True:
            another_author = self._get_user_info(b'commit', b'author', False)
            if another_author is not None:
                more_authors.append(another_author)
            else:
                break
        committer = self._get_user_info(b'commit', b'committer')
        message = self._get_data(b'commit', b'message')
        from_ = self._get_from()
        merges = []
        while True:
            merge = self._get_merge()
            if merge is not None:
                # while the spec suggests it's illegal, git-fast-export
                # outputs multiple merges on the one line, e.g.
                # merge :x :y :z
                these_merges = merge.split(b' ')
                merges.extend(these_merges)
            else:
                break
        properties = {}
        while True:
            name_value = self._get_property()
            if name_value is not None:
                name, value = name_value
                properties[name] = value
            else:
                break
        return commands.CommitCommand(ref, mark, author, committer, message,
            from_, merges, list(self.iter_file_commands()), lineno=lineno,
            more_authors=more_authors, properties=properties)