If $hash_parent is defined, shortlog now limits the list of commits at
those between $hash_parent (exclusive) and $hash (inclusive).
Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com> Acked-by: Petr Baudis <pasky@suse.cz> Signed-off-by: Shawn O. Pearce <spearce@spearce.org>