Re: rebase backend change interaction with rebase.abbreviateCommands and autostash

>
> Hi,
>
> I just had a bizarre error related to the change in the merge backend. I don't have a full reproducer yet but the following sequence should hopefully give some hints.
>
> In my config for this repo I have `pull.rebase=true` and in my global config I have
>
> [rebase]
>         abbreviateCommands = true
>         autoSquash = true
>
> Also my local `master` branch tracks `origin/master`.
>
> If `master` on `origin` is at the same commit as my local master branch, and I have some uncommitted change so I must use `--autostash`:
>
> ```
> $ git pull --autostash
> Created autostash: 6a6c685
> Applied autostash.
> error: nothing to do
>
> $ git pull --autostash --apply
> # or right that just works for `git rebase`
> ```
>
> Now if `master` on `origin` is ahead of my local `master`:
>
> ```
> $ git pull --autostash
> remote: Counting objects: 6, done.
> remote: Compressing objects: 100% (6/6), done.
> remote: Total 6 (delta 4), reused 0 (delta 0)
> Unpacking objects: 100% (6/6), 758 bytes | 6.00 KiB/s, done.
> From <redacted>
>    42033de..a3d6919  master     -> origin/master
> Created autostash: cdae23b
> Applied autostash.
> error: nothing to do
>
> Now if I stash manually and re-pull:
>
> $ git stash push -m "test rebase merge"
> Saved working directory and index state On master: test rebase merge
> $ git pull
> Updating 42033de..a3d6919
> Fast-forward
>  .gitconfig | 1 +
>  .tmux.conf | 2 +-
>  2 files changed, 2 insertions(+), 1 deletion(-)
> error: nothing to do
> $ git pull
> Already up to date.
> error: nothing to do
> $ git -c rebase.abbreviateCommands=0 pull
> Already up to date.
> Successfully rebased and updated refs/heads/master.
>
> Note: I also got  "error: nothing to do"  when manually running the commands in the last test of t/t5572-pull-submodule.sh ("branch has no merge base with remote-tracking counterpart') (and again using `git -c rebase.abbreviateCommands=0` succeeded).
>
> Cheers,
> Philippe.