combine-diff: fix appending at the tail of a list.

... and use the established pattern of tail initialized to point
at the head pointer for an empty list, and updated to point at
the next pointer field of the item at the tail when appending.

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

combine-diff.c

@@ -29,7 +29,7 @@ static struct path_list *intersect_paths(struct path_list *curr,
 	int i;
 
 	if (!n) {
-		struct path_list *list = NULL, *tail = NULL;
+		struct path_list *list = NULL, **tail = &list;
 		for (i = 0; i < q->nr; i++) {
 			int len;
 			const char *path;
@@ -46,12 +46,8 @@ static struct path_list *intersect_paths(struct path_list *curr,
 			p->next = NULL;
 			memcpy(p->sha1, q->queue[i]->two->sha1, 20);
 			memcpy(p->parent_sha1[n], q->queue[i]->one->sha1, 20);
-			if (!tail)
-				list = tail = p;
-			else {
-				tail->next = p;
-				p = tail;
-			}
+			*tail = p;
+			tail = &p->next;
 		}
 		return list;
 	}
@@ -212,10 +208,7 @@ static void append_lost(struct sline *sline, int n, const char *line)
 	lline->parent_map = this_mask;
 	memcpy(lline->line, line, len);
 	lline->line[len] = 0;
-	if (sline->lost_head)
-		*(sline->lost_tail) = lline;
-	else
-		sline->lost_head = lline;
+	*sline->lost_tail = lline;
 	sline->lost_tail = &lline->next;
 }
 
@@ -433,6 +426,7 @@ static void show_combined_diff(struct path_list *elem, int num_parent,
 	sline[0].bol = result;
 	for (lno = 0, cp = result; cp - result < size; cp++) {
 		if (*cp == '\n') {
+			sline[lno].lost_tail = &sline[lno].lost_head;
 			sline[lno].len = cp - sline[lno].bol;
 			sline[lno].flag = (1UL<<num_parent) - 1;
 			lno++;
@@ -441,6 +435,7 @@ static void show_combined_diff(struct path_list *elem, int num_parent,
 		}
 	}
 	if (result[size-1] != '\n') {
+		sline[cnt-1].lost_tail = &sline[cnt-1].lost_head;
 		sline[cnt-1].len = size - (sline[cnt-1].bol - result);
 		sline[cnt-1].flag = (1UL<<num_parent) - 1;
 	}