Fix a buffer overflow in diff -r.

We were doing two 32-byte memset()s instead of two 16-byte memset()s.
'dir' referred to the instance (array of 2) and not the struct type.

Add some test coverage for diff, including a case that hit this bug.

The bug was found by running cp.test under AddressSanitizer, since it
happens to use diff.

tests/diff.test

@@ -0,0 +1,30 @@
+#!/bin/bash
+
+#testing "name" "command" "result" "infile" "stdin"
+
+seq 10 > left
+seq 11 > right
+
+expected='--- left
++++ right
+@@ -8,3 +8,4 @@
+ 8
+ 9
+ 10
++11
+'
+# Hm this only gives unified diffs?
+testing "simple" "diff left right" "$expected" "" ""
+
+
+expected='--- tree1/file
++++ tree2/file
+@@ -1 +1 @@
+-foo
++food
+'
+mkdir -p tree1 tree2
+echo foo > tree1/file
+echo food > tree2/file
+
+testing "simple" "diff -r tree1 tree2 |tee out" "$expected" "" ""

toys/pending/diff.c

@@ -59,7 +59,7 @@ struct diff {
   long a, b, c, d, prev, suff;
 };
 
-static struct dir {
+static struct dir_t {
   char **list;
   int nr_elm;
 } dir[2];
@@ -69,7 +69,7 @@ struct candidate {
   struct candidate *prev, *next;
 };
 
-static struct file {
+static struct file_t {
   FILE *fp;
   int len;
 } file[2];
@@ -797,7 +797,7 @@ void diff_main(void)
 
   if (S_ISDIR(st[0].st_mode) && S_ISDIR(st[1].st_mode)) {
     for (j = 0; j < 2; j++) {
-      memset(&dir[j], 0, sizeof(dir));
+      memset(&dir[j], 0, sizeof(struct dir_t));
       dirtree_flagread(files[j], DIRTREE_SYMFOLLOW, list_dir);
       dir[j].nr_elm = TT.size; //size updated in list_dir
       qsort(&(dir[j].list[1]), (TT.size - 1), sizeof(char*), cmp);