Speed up callers: and called-by: queries by replacing SQL subqueries with joins.
Fix incorrect condition in SQL (callers.callerid = target.id) that was
preventing indirect call results from appearing when searching with callers:.
Merge branch 'master' of github.com:abbeyj/dxr into callers-calledby-sql
Merge speed-ups for callers: and called-by: queries. Fix #124.