Use a common function to get the pretty name of refs

Use a common function to get the pretty name of refs

The result should be consistent between fetch and push, so we ought to
use the same code in both cases, even though it's short.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>