t: use test_might_fail for diff and grep

Some tests run diff or grep to produce an output, and then
compare the output to an expected value. We know the exit
code we expect these processes to have (e.g., grep yields 0
if it produced output and 1 otherwise), so it would not make
the test wrong to look for it. But the difference between
their output and the expected output (e.g., shown by
test_cmp) is much more useful to somebody debugging the test
than the test just bailing out.

These tests break the &&-chain to skip the exit-code check
of the process. However, we can get the same effect by using
test_might_fail. Note that in some cases the test did use
"|| return 1", which meant the test was not wrong, but it
did fool --chain-lint.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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