When merging hunks, keep all FROM lines before corresponding TO lines.

Fixes crash when merging
	 common 1
	-deleted 1
	+inserted 1
	 deleted 2
	 common 2
and
	 common 1
	 inserted 1
	-deleted 2
	+inserted 2
	+inserted 3
	 common 2
These should merge to
	 common 1
	-deleted 1
	-deleted 2
	+inserted 1
	+inserted 2
	+inserted 3
	 common 2
but before this fix the result was
	 common 1
	-deleted 1
	+inserted 1
	-deleted 2
	+inserted 2
	+inserted 3
	 common 2

I can't help feeling there should be a cleaner fix, a way to generate
the merged hunk in the right order to begin with.