extsearchidx: simplify reindex code paths

extsearchidx: simplify reindex code paths

Since we're inside a Xapian transaction, calling ->index_raw
followed by ->shard_add_eidx_info calls on the same docid
doesn't seem to hurt indexing performance.  It definitely
reduces FS read traffic and IPC from git at the cost of some
more IPC between the parent and workers.  Nevertheless, the code
and FD reductions seem worth it.