get_revision(): do not dig deeper when we know we are at the end.
get_revision(): do not dig deeper when we know we are at the end.

This resurrects the special casing for "rev-list -n 1" which
avoided reading parents unnecessarily.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git