Merge 'search_related_prefetch' into 'trunk'

r6815@Thesaurus (orig r6814):  ribasushi | 2009-06-28 10:32:42 +0200
Branch to explore double joins on search_related
r6816@Thesaurus (orig r6815):  ribasushi | 2009-06-28 10:34:16 +0200
Thetest case that started it all
r6817@Thesaurus (orig r6816):  ribasushi | 2009-06-28 10:35:11 +0200
The proposed fix (do not add an extra join if it is already present in the topmost join)
r6818@Thesaurus (orig r6817):  ribasushi | 2009-06-28 11:04:26 +0200
Minor omission
r6819@Thesaurus (orig r6818):  ribasushi | 2009-06-28 11:07:33 +0200
Adjust a couple of tests for new behavior (thus all of this might be backwards incompatible to the point of being useless):
The counts in t/90join_torture.t are now 5*3, not 5*3*3, as a second join is not induced by search_related
The raw sql scan in t/prefetch/standard.t is just silly, won't even try to understand it
Just to maintain the TreeLike folding, I add a 3rd children join which was inserted by search_related before the code changes