Fix up diffcore-rename scoring

The "score" calculation for diffcore-rename was totally broken.

It scaled "score" as

	score = src_copied * MAX_SCORE / dst->size;

which means that you got a 100% similarity score even if src and dest were
different, if just every byte of dst was copied from src, even if source
was much larger than dst (eg we had copied 85% of the bytes, but _deleted_
the remaining 15%).

That's clearly bogus. We should do the score calculation relative not to
the destination size, but to the max size of the two.

This seems to fix it.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diffcore-rename.c

@@ -133,7 +133,7 @@ static int estimate_similarity(struct diff_filespec *src,
 	 * match than anything else; the destination does not even
 	 * call into this function in that case.
 	 */
-	unsigned long delta_size, base_size, src_copied, literal_added;
+	unsigned long max_size, delta_size, base_size, src_copied, literal_added;
 	unsigned long delta_limit;
 	int score;
 
@@ -144,9 +144,9 @@ static int estimate_similarity(struct diff_filespec *src,
 	if (!S_ISREG(src->mode) || !S_ISREG(dst->mode))
 		return 0;
 
-	delta_size = ((src->size < dst->size) ?
-		      (dst->size - src->size) : (src->size - dst->size));
+	max_size = ((src->size > dst->size) ? src->size : dst->size);
 	base_size = ((src->size < dst->size) ? src->size : dst->size);
+	delta_size = max_size - base_size;
 
 	/* We would not consider edits that change the file size so
 	 * drastically.  delta_size must be smaller than
@@ -174,12 +174,10 @@ static int estimate_similarity(struct diff_filespec *src,
 	/* How similar are they?
 	 * what percentage of material in dst are from source?
 	 */
-	if (dst->size < src_copied)
-		score = MAX_SCORE;
-	else if (!dst->size)
+	if (!dst->size)
 		score = 0; /* should not happen */
 	else
-		score = src_copied * MAX_SCORE / dst->size;
+		score = src_copied * MAX_SCORE / max_size;
 	return score;
 }