Restrict ref-like names immediately below $GIT_DIR

Restrict ref-like names immediately below $GIT_DIR

We have always dwimmed the user input $string into a ref by first looking
directly inside $GIT_DIR, and then in $GIT_DIR/refs, $GIT_DIR/refs/tags,
etc., and that is what made

    git log HEAD..MERGE_HEAD

work correctly. This however means that

    git rev-parse config
    git log index

would look at $GIT_DIR/config and $GIT_DIR/index and see if they are valid
refs.

To reduce confusion, let's not dwim a path immediately below $GIT_DIR that
is not all-caps.

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