tests: correct "does reflog exist?" tests

These two tests weren't about how "git reflog show <branch>" exits when
there is no reflog, but were about "checkout" and "branch" create or not
create reflog when creating a new <branch>.  Update the tests to check
what we are interested in, using "git rev-parse --verify".

Also lose tests based on "test -f .git/logs/refs/heads/<branch>" from
nearby, to avoid exposing this particular implementation detail
unnecessarily.

Signed-off-by: Junio C Hamano <gitster@pobox.com>