This is a stab at tweaking the new batch-tag dump/restore format to be more amenable to batch tagging. Currently, the "query" part of each line isn't really a Xapian query; it's a hex-encoded message ID prefixed with "id:". This is fine for the very limited case of dump/restore, but it extends poorly to the general queries allowed by batch tagging. However, Xapian already has a perfectly good quoting syntax. This series switches the batch-tag format to use regular Xapian queries, using only regular Xapian quoting syntax, so that we don't have to invent yet another quoting syntax for batch tagging.