searchidx: fix incremental index with indexlevel=basic on v1
We were reindexing the full history every invocation of -index
when Xapian was not used because we were incorrectly relying on
'last_commit' metadata stored in Xapian.
Rewrite the indexing logic to be less confusing while we're
at it, since we rely on `git merge-base --is-ancestor' nowadays.
Furthermore, we need to handle message removals from the
overview index correctly when Xapian is not in use.
Co-authored-by: Eric W. Biederman <ebiederm@xmission.com>