merge-base: die with an error message if not passed a commit ref

merge-base: die with an error message if not passed a commit ref

Before this patch "git merge-base" just exited with error code 1
and without an error message in case it was passed a ref to an
object that is not a commit (for example a tree).

This patch makes it "die" in this case with an error message.

While at it, this patch also refactors the code to get the
commit reference from an argument into a new
"get_commit_reference" function.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>