public_inbox: Fix email threads without an available root message - #369

Some type hints were set to return a given type, while the function was
actually returning that type, or a None object.
Fix the type hints when relevant.
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Some messages will not be perfect, and will have inconsistent or missing
headers.
We were mostly handling them fine, but didn't log anything when that was
happening, making debugging issues fairly difficult. Let's improve the
logging a bit.
Signed-off-by: Maxime Ripard <mripard@kernel.org>

Commit af2b8ac ("Fix pylint R1710") made __fetch_thread_root()
return the original message argument if no thread root could be found.
However, and even though it wasn't really handled yet, the intent was
always to return None in such a case.
Restore the previous behaviour, but in an explicit way this time, and
we'll handle it in the callers in the next commit.
Signed-off-by: Maxime Ripard <mripard@kernel.org>

A message can have an In-Reply-To header but the message it points to
might not be available for some reason.
In such a case, the Message.parent_id() method will return a proper
value, but __fetch_thread_root() on that message might not and would
return None.
Let's handle that case by returning the latest message we found as the
root message.
Signed-off-by: Maxime Ripard <mripard@kernel.org>