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>