[Toybox] [PATCH] Fix a buffer overflow in diff -r.

[Andy Chu]

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.
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