t5703: simplify one-time-sed generation logic

t5703: simplify one-time-sed generation logic

In inconsistency(), we had two `git rev-parse` invocations in the
upstream of a pipe within a command substitution. In case this
invocation ever failed, its exit code would be swallowed up and we would
not know about it.

Pull the command substitutions out into variable assignments so that
their return codes are not lost.

Drop the pipe into `tr` because the $(...) substitution already takes
care of stripping out newlines, so the `tr` invocations in the code are
superfluous.

Finally, given the way the tests actually employ "one-time-sed" via
$(cat one-time-sed) in t/lib-httpd/apply-one-time-sed.sh, convert the
`printf` into an `echo`. This makes it consistent with the final "server
loses a ref - ref in want" test, which does use `echo` rather than
`printf`.

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