Allow rebase to run if upstream is completely merged

Allow rebase to run if upstream is completely merged

Consider this history:

  o--o-...-B          <- origin
      \     \
       x--x--M--x--x  <- master

In this situation, rebase considers master fully up-to-date and would
not do anything. However, if there were additional commits on origin,
the rebase would run and move the commits x on top of origin.

Here we change rebase to short-circuit out only if the history since origin
is strictly linear. Consequently, the above as well as a history like this
would be linearized:

  o--o               <- origin
      \
       x--x
        \  \
         x--M--x--x  <- master

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>