fast-import: use pointer-to-pointer to keep list tail

This is shorter, idiomatic, and it means the compiler does
not get confused about whether our "e" pointer is valid,
letting us drop the "e = e" hack.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

fast-import.c

@@ -2613,7 +2613,7 @@ static int parse_from(struct branch *b)
 
 static struct hash_list *parse_merge(unsigned int *count)
 {
-	struct hash_list *list = NULL, *n, *e = e;
+	struct hash_list *list = NULL, **tail = &list, *n;
 	const char *from;
 	struct branch *s;
 
@@ -2641,11 +2641,9 @@ static struct hash_list *parse_merge(unsigned int *count)
 			die("Invalid ref name or SHA1 expression: %s", from);
 
 		n->next = NULL;
-		if (list)
-			e->next = n;
-		else
-			list = n;
-		e = n;
+		*tail = n;
+		tail = &n->next;
+
 		(*count)++;
 		read_next_command();
 	}