Don't use ResetMixed for "Unstage All" button when the commit is a merge

[kleee]

Hi,

The "Unstage All" command in the commit form is applying a ResetMixed operation. But, this operation will delete the Merge information.

I suggest to disable the button when it's a merge, or warn the user that this will "broke" the merge (ex with a popup : are you sure...).
In fact, we got some"wrong" merge due to this command in our company, and it's quite difficult to repair afterward.
I'm can try implement this, if you're agreed with the feature.

Regards,