Option for "--squash --ff-only" merge strategy

Squash, fast-forward only (--squash --ff-only): If the source branch is out of date with the target branch, reject the merge request. Otherwise, combine all commits into one new non-merge commit on the target branch

Use case :
A pull request rebased on master branch contains one feature commit plus some code improvement introduced by the review process.

This give us the Following configuration :

#!git

                   D---D'---D''  Feature-XYZ
                 /
    A---B---C  master

that should be merged as

#!git

    A---B---C ---D master