t3310: avoid hiding failures from rev-parse in command substitutions

t3310: avoid hiding failures from rev-parse in command substitutions

Running `git` commands inside command substitutions like

    test "$(git rev-parse A)" = "$(git rev-parse B)"

can hide failures from the `git` invocations and provide little
diagnostic information when `test` fails.

Use `test_cmp` when comparing against a stored expected value so
mismatches show both expected and actual output. Use `test_cmp_rev`
when comparing two revisions. These helpers produce clearer failure
output, making it easier to understand what went wrong.

Suggested-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Francesco Paparatto <francescopaparatto@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>