for-each-ref: refactor get_short_ref function

for-each-ref: refactor get_short_ref function

This function took a "refinfo" object which is unnecessarily
restrictive; it only ever looked at the refname field. This
patch refactors it to take just the ref name as a string.

While we're touching the relevant lines, let's give it
consistent memory semantics. Previously, some code paths
would return an allocated string and some would return the
original string; now it will always return a malloc'd
string.

This doesn't actually fix a bug or a leak, because
for-each-ref doesn't clean up its memory, but it makes the
function a lot less surprising for reuse (which will
happen in a later patch).

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