t4002: fix "diff can read from stdin" syntax

t4002: fix "diff can read from stdin" syntax

I noticed this test was producing output like

```
t4002-diff-basic.sh: test_expect_successdiff can read from stdin: not found
```

which is rather odd. Investigation shows an error of shell syntax:
foo'abc' is the same as fooabc to the shell. Perhaps obviously, this is
not a valid command for the test.

I am surprised this doesn't count as an error in the test, but that
accounts for it going unnoticed.

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>