devel/review: make the range argument optional

Rather than calling devel/review with from..to target we can just use
target, and assume target..HEAD as the range. This makes it a lot
easier, as now you just have to say something like "devel/review
devel-2.1" if you're already on the branch you want to commit, and want
to commit it to devel-2.1.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>