xdl_change_compact(): introduce the concept of a change group

The idea of xdl_change_compact() is fairly simple:

* Proceed through groups of changed lines in the file to be compacted,
  keeping track of the corresponding location in the "other" file.

* If possible, slide the group up and down to try to give the most
  aesthetically pleasing diff. Whenever it is slid, the current location
  in the other file needs to be adjusted.

But these simple concepts are obfuscated by a lot of index handling that
is written in terse, subtle, and varied patterns. I found it very hard
to convince myself that the function was correct.

So introduce a "struct group" that represents a group of changed lines
in a file. Add some functions that perform elementary operations on
groups:

* Initialize a group to the first group in a file
* Move to the next or previous group in a file
* Slide a group up or down

Even though the resulting code is longer, I think it is easier to
understand and review. Its performance is not changed
appreciably (though it would be if `group_next()` and `group_previous()`
were not inlined).

...and in fact, the rewriting helped me discover another bug in the
--compaction-heuristic code: The update of blank_lines was never done
for the highest possible position of the group. This means that it could
fail to slide the group to its highest possible position, even if that
position had a blank line as its last line. So for example, it yielded
the following diff:

    $ git diff --no-index --compaction-heuristic a.txt b.txt
    diff --git a/a.txt b/b.txt
    index e53969f..0d60c5fe 100644
    --- a/a.txt
    +++ b/b.txt
    @@ -1,3 +1,7 @@
     1
     A
    +
    +B
    +
    +A
     2

when in fact the following diff is better (according to the rules of
--compaction-heuristic):

    $ git diff --no-index --compaction-heuristic a.txt b.txt
    diff --git a/a.txt b/b.txt
    index e53969f..0d60c5fe 100644
    --- a/a.txt
    +++ b/b.txt
    @@ -1,3 +1,7 @@
     1
    +A
    +
    +B
    +
     A
     2

The new code gives the bottom answer.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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