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>

1 files changed, 1 insertions, 0 deletions

diff --git a/MANIFEST b/MANIFEST
index 28300e0c..1da40a90 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -198,6 +198,7 @@ t/httpd.t
 t/hval.t
 t/import.t
 t/inbox.t
+t/indexlevels-mirror.t
 t/init.t
 t/linkify.t
 t/main-bin/spamc