t6018: flesh out empty input/output rev-list tests

t6018: flesh out empty input/output rev-list tests

In 751a2ac6e (rev-list --exclude: tests, 2013-11-01), we
added a few tests for handling "empty" inputs with rev-list
(i.e., where the user gave us some pattern but it turned out
not to queue any objects for traversal), all of which were
marked as failing.

In preparation for working on this area of the code, let's
give each test a more descriptive name. Let's also include
one more case which we should cover: feeding a --glob
pattern that doesn't match anything.

We can also drop the explanatory comment; we'll be
converting these to expect_success in the next few patches,
so the discussion isn't necessary.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>