In many places, we refer to objects via their SHA-1s. Let's abstract
that into a function.
For the moment, it does nothing else than what we did previously: print
out the 40-digit hex string. But that will change over the course of the
next patches.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>