Make list assignment respect foreach aliasing

Make list assignment respect foreach aliasing

See ff2a62e0c8 for the explanation.  The bug fix in that commit did
not apply to foreach’s aliasing.

In short, ($a,$b)=($c,$d) needs to account for whether two of those
variable names could be referring to the same variable.

This commit causes the test suite to exercise a code path in scope.c
added by ff2a62e0c8, which turned out to be buggy.  (I forgot to test
it at the time.)