range-set: fix sort_and_merge_range_set() corner case bug

When handed an empty range_set (range_set.nr == 0),
sort_and_merge_range_set() incorrectly sets range_set.nr to 1 at exit.
Subsequent range_set functions then access the bogus range at element
zero and crash or throw an assertion failure. Fix this bug.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Acked-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: sunshineco

line-log.c

@@ -110,12 +110,12 @@ static void range_set_check_invariants(struct range_set *rs)
 static void sort_and_merge_range_set(struct range_set *rs)
 {
 	int i;
-	int o = 1; /* output cursor */
+	int o = 0; /* output cursor */
 
 	qsort(rs->ranges, rs->nr, sizeof(struct range), range_cmp);
 
-	for (i = 1; i < rs->nr; i++) {
-		if (rs->ranges[i].start <= rs->ranges[o-1].end) {
+	for (i = 0; i < rs->nr; i++) {
+		if (o > 0 && rs->ranges[i].start <= rs->ranges[o-1].end) {
 			if (rs->ranges[o-1].end < rs->ranges[i].end)
 				rs->ranges[o-1].end = rs->ranges[i].end;
 		} else {