ngRepeat: insert end comment only after last element, not after each iterated item

This caused a subtle bug in angular-sortable directive: when moving elements
between lists, previousNode was a comment, which doesn't have the .parentNode
attribute so the $animate.enter function tried to call insertBefore of a null.

Moreover, it looks bad that 'end ngRepeat' comment appears after every iterated
node, it should be just at the end of the repeat block.