t: teach test_cmp_rev to accept ! for not-equals

t: teach test_cmp_rev to accept ! for not-equals

In the case where we are using test_cmp_rev() to report not-equals, we
write `! test_cmp_rev`. However, since test_cmp_rev() contains

	r1=$(git rev-parse --verify "$1") &&
	r2=$(git rev-parse --verify "$2") &&

`! test_cmp_rev` will succeed if any of the rev-parses fail. This
behavior is not desired. We want the rev-parses to _always_ be
successful.

Rewrite test_cmp_rev() to optionally accept "!" as the first argument to
do a not-equals comparison. Rewrite `! test_cmp_rev` to `test_cmp_rev !`
in all tests to take advantage of this new functionality.

Also, rewrite the rev-parse logic to end with a `|| return 1` instead of
&&-chaining into the rev-comparison logic. This makes it obvious to
future readers that we explicitly intend on returning early if either of
the rev-parses fail.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>