Tweak break/merge score to adjust to the new delta generation code.

Tweak break/merge score to adjust to the new delta generation code.

This lowers the default merge threshold score to 75% from
earlier 80%.  The break threshold stays the same at 50% for now,
but we might want to revisit it (and the rename detection limit
as well).

 * break score: this much edit (both insertion of new material
   and deletion of old material) needs to be there in the file
   before we consider this _might_ be a rewrite and break the
   filepair.

 * merge score: after a filepair is broken by the above criteria
   and goes through rename detection, if their pieces did not
   match with other files as rename/copy, we merge them back
   into one as if nothing happened.  If the filepair had at
   least this much deletion of old material, however, we say
   this is completely rewritten with dissimilarity index X% when
   we do so.

The updated delta code by Nico is so good that what we earlier
thought to be complete rewrite now reuses a lot more from the
source material (reducing the counted "delete"), so this
adjustment is needed to keep the perceived behaviour similar to
what we had earlier.

Signed-off-by: Junio C Hamano <junkio@cox.net>