reflog tests: add missing "git reflog exists" tests

reflog tests: add missing "git reflog exists" tests

There were a few "git reflog exists" tests scattered over the test
suite, but let's consolidate the testing of the main functionality
into a new test file. This makes it easier to run just these tests
during development.

To do that amend and extend an existing test added in
afcb2e7a3b8 (git-reflog: add exists command, 2015-07-21). Let's use
"test_must_fail" instead of "!" (in case it segfaults), and test for
basic usage, an unknown option etc.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>