Don't force the user to specify more than one revision parameter,
thus making git-shortlog behave more like git-log.
'git-shortlog master' will now produce the expected results; the
other end of the range simply is the (oldest) root commit.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx> Signed-off-by: Junio C Hamano <junkio@cox.net>