In the past, we dis-allowed rebases using the interactive backend from
performing a fast-forward to short-circuit the rebase operation. This
made sense for explicitly interactive rebases and some implicitly
interactive rebases, but certainly became overly stringent when the
merge backend was re-implemented via the interactive backend.
Just as the am-based rebase has always had to disable the fast-forward
based on a variety of conditions or flags (e.g. --signoff, --whitespace,
etc.), we need to do the same but now with a few more options. However,
continuing to use REBASE_FORCE for tracking this is problematic because
the interactive backend used it for a different purpose. (When
REBASE_FORCE wasn't set, the interactive backend would not fast-forward
the whole series but would fast-forward individual "pick" commits at the
beginning of the todo list, and then a squash or something would cause
it to start generating new commits.) So, introduce a new
allow_preemptive_ff flag contained within cmd_rebase() and use it to
track whether we are going to allow a pre-emptive fast-forward that
short-circuits the whole rebase.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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