group_by_target does not properly combine target groups if statements
are presented in the order:
({A}, statement1)
({B}, statement2)
({A, B}, statement3)
which returns groups:
({A, B}, [statement1, statement3])
({B}, [statement2])
This patch fixes group_by_target such that the resulting group is:
({A, B}, [statement1, statement2, statement3])
16ebe41028