[PATCH 7 of 7 v2] bdiff: give slight preference to removing trailing lines

[Jun Wu]

Excerpts from Augie Fackler's message of 2016-11-17 12:42:26 -0500:
> My own cursory perfbdiff runs suggest this is a perf wash (using
> `perfbdiff -m 3041e4d59df2` in the mozilla repo). Queued. Thanks!

I'd mention this series changes the behavior of the diff output. The
difference was caught by fastannotate test.

See the below table (old: e1d6aa0e4c3a, new: 8836f13e3c5b):

   a | b | old | new
  --------------------
   a | a |  a  | -a
   a | z | +z  |  a
   a | a |  a  | +z
     |   | -a  |  a
  --------------------
   a | a |     a
   a | a |     a
   a |   |    -a

I think we would always prefer putting deletions at the end, to be consistent.