Use `defaultdict` where applicable

[mkondratek]

Most likely for the following:

• down_branches: Dict[str, List[str]] = {} # TODO (#): default dict with []
• up_branch: Dict[str, str] = {} # TODO (#): default dict with None
• commit_sha_by_revision_cached: Optional[Dict[str, Optional[str]]] = None # TODO (#): default dict with None
• committer_unix_timestamp_by_revision_cached: Optional[Dict[str, int]] = None # TODO (#): default dict with 0