rev-parse: fix parent shorthands with --symbolic

rev-parse: fix parent shorthands with --symbolic

The try_parent_shorthands() function shows each parent via
show_rev(). We pass the correct parent sha1, but our "name"
parameter still points at the original refname. So asking
for a regular rev-parse works fine (it prints the sha1s),
but asking for the symbolic name gives nonsense like:

    $ git rev-parse --symbolic HEAD^-1
    HEAD
    ^HEAD

which is always an empty set of commits. Asking for "^!" is
likewise broken, with the added bonus that its prints ^HEAD
for _each_ parent. And "^@" just prints HEAD repeatedly.

Arguably it would be correct to just pass NULL as the name
here, and always get the parent expressed as a sha1. The
"--symbolic" documentaton claims only "as close to the
original input as possible", and we certainly fallback to
sha1s where necessary. But it's pretty easy to generate a
symbolic name on the fly from the original.

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