<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
+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.