diffcore-break: micro-optimize by avoiding delta between identical files.

We did not check if we have the same file on both sides when
computing break score.  This is usually not a problem, but if
the user said --find-copies-harde with -B, we ended up trying a
delta between the same data even when we know the SHA1 hash of
both sides match.

Signed-off-by: Junio C Hamano <junkio@cox.net>
(cherry picked from aeecd23ae2785a0462d42191974e9d9a8e439fbe commit)