Re: [PATCH] rebase -i: avoid checking out $branch when possible

[Junio C Hamano <gitster@pobox.com>]

Thomas Rast <trast@inf.ethz.ch> writes:

>>  - The user told us not to do the rebase by making insn sheet empty.  In
>>    other words, this is "aborting the entire rebase", so ideally it
>>    should come back to the state before the user ran "git rebase"
>>    command (i.e. where she was before we switched to <branch>).
>> 
>>    I do not think this ideal behaviour is something neither batch or
>>    interactive rebase has traditionally implemented, but I can see how
>>    we can sell this as a bugfix to the end users.
>
> That's a convincing argument, so let's make it so.
>
>>  - It turns out that everything is already contained and there is
>>    nothing to apply, i.e. after this sequence:
>> 
>> 	git checkout branch
>> 	git checkout $onto_or_merge_base_between_base_and_branch
>> 
>>    we find out that "git cherry $onto_or_merge_base branch" is empty.
>
> Is there a command missing here?  This alone does not make them the
> same, perhaps you meant some resetting.

Actually that was phrased rather poorly.  At least "after this sequence"
should have been "during this sequence inside the implementation of
'rebase -i'".  In other words, the scenario is still where the user
typed:

  git rebase [-i] [--onto $onto] $base $branch

which is defined to be equivalent to

  git checkout $branch && git rebase [-i] [--onto $onto] $base

The first half of the equivalent command succeeds.  We are on $branch.
The second half finds there is nothing to be done.

I think the current code (and probably with your patch) does this in
such a case:

has_action "$todo" ||
	die_abort "Nothing to do"

and the die happens on $branch before we detach (iow, between the two
checkouts), and the user should end up on $branch.  Unlike the case
where the user told us to abort the entire rebase, which we agreed that
we should come back to the original branch above, I think in this case
we should not nullify the first "checkout $branch".