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);