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.