searchidx: regenerate and avoid article number gaps on full index
Some messages to git@vger went missing from Msgmap from old bugs
and became inaccessible via NNTP. Forcing NNTP article numbers
when the overview DB came about made the problem more visible when
reindexing old (v1) repositories as all removed spam messages
took up AUTOINCREMENT numbers again before they were removed.
Having large gaps in NNTP article numbers is not good since it
throws off NNTP clients. This does NOT prevent NNTP clients from
seeing some messages twice, but is better than having them
miss several messages entirely.
We also avoid depending on --reverse in git-log, as
git requires storing an entire commit list in memory for
--reverse, so it's cheaper to store only deleted blobs in the %D
hash since they do not live long.