[BUG-ish] diff compaction heuristic false positive

[Jeff King <peff@peff.net>]

I found a false positive with the new compaction heuristic in v2.9:

-- >8 --

# start with a simple file
cat >file.rb <<\EOF
def foo
  do_foo_stuff()

  common_ending()
end
EOF

git add file.rb
git commit -m base

# and then add another function with a similar ending
cat >>file.rb <<\EOF

def bar
  do_bar_stuff()

  common_ending()
end
EOF

-- 8< --

I get this rather unfortunate diff:

    $ git diff
    diff --git a/file.rb b/file.rb
    index bd9d1cb..67fbeba 100644
    --- a/file.rb
    +++ b/file.rb
    @@ -1,5 +1,11 @@
     def foo
       do_foo_stuff()
     
    +  common_ending()
    +end
    +
    +def bar
    +  do_bar_stuff()
    +
       common_ending()
     end

but without the compaction heuristic (or with an older git), I get:

    $ git -c diff.compactionHeuristic=false diff
    diff --git a/file.rb b/file.rb
    index bd9d1cb..67fbeba 100644
    --- a/file.rb
    +++ b/file.rb
    @@ -3,3 +3,9 @@ def foo
     
       common_ending()
     end
    +
    +def bar
    +  do_bar_stuff()
    +
    +  common_ending()
    +end

:( The problem is that the common bits are separated from the
interesting bits by a blank line. This is simplified from a real-world
example, but I think you could come up with the same example in C, like:

  if (foo) {
        do_foo();

        something_else();
  }

-Peff