diffcore-break: don't divide by zero

When the source file is empty, the calculation of the merge score
results in a division by zero.  In the situation:

     == preimage ==             == postimage ==

     F (empty file)             F (a large file)
                                E (a new empty file)

it does not make sense to consider F->E as a rename, so it is better not
to break the pre- and post-image of F.

Bail out early in this case to avoid hitting the divide-by-zero.  This
causes the merge score to be left at zero.

Signed-off-by: John Keeping <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

diffcore-break.c

@@ -68,6 +68,9 @@ static int should_break(struct diff_filespec *src,
 	if (max_size < MINIMUM_BREAK_SIZE)
 		return 0; /* we do not break too small filepair */
 
+	if (!src->size)
+		return 0; /* we do not let empty files get renamed */
+
 	if (diffcore_count_changes(src, dst,
 				   &src->cnt_data, &dst->cnt_data,
 				   0,