When comparing hunks, return true if they are functionally equal even if they're not equal in detail.

For example, these two hunks should match, because their old and new
texts are equal, even though they are different in detail:

	 one			 one
	-two			-two
	-line			+two bis
	-unmatched		 line
	+two bis		-unmatched
	 line			-line
	-three			-three
	+three bis		+three bis
	 four			 four