t1515: add tests for rev-parse out-of-repo helpers

t1515: add tests for rev-parse out-of-repo helpers

The git-rev-parse command is a dumping ground for helpers
that let scripts make various queries of git. Many of these
are conceptually independent of being inside a git
repository.

With the exception of --parseopt, we do not directly test
most of these features in our test suite. Let's give them
some basic sanity checks, which reveals that some of them
have been broken for some time when run from outside a
repository.

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