Introduce --base=auto to record the base commit info automatically, the
base_commit will be the merge base of tip commit of the upstream branch
and revision-range specified in cmdline.
Helped-by: Junio C Hamano <gitster@pobox.com> Helped-by: Wu Fengguang <fengguang.wu@intel.com> Signed-off-by: Xiaolong Ye <xiaolong.ye@intel.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>