`public-inbox-index --reindex' could cause NNTP article number
gaps to form when it also has to deal with new,
never-before-seen commits in mirrors running off `git fetch'.
Fix this by running two distinct invocations of ->index_sync;
once to only reindex old commits, and a second time to index
new commits.
This does not appear to be a problem on v1 at the moment,
but I'll need more time to analyze this.