in heads.ml, at least turn the Not_found of #2608 into assert false
in heads.ml, at least turn the Not_found of #2608 into assert false

git-svn-id: 44dc50bdcc0f5d1a212c497508dc9452e0b35e2b@14550 85f007b7-540e-0410-9357-904b9bb8a0f7

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