--onto option is not specified, the starting point is
<upstream>. May be any valid commit, and not just an
existing branch name.
++
+As a special case, you may use "A...B" as a shortcut for the
+merge base of A and B if there is exactly one merge base. You can
+leave out at most one of A and B, in which case it defaults to HEAD.
<upstream>::
Upstream branch to compare against. May be any valid commit,