Ability to fixup into the above commit

**Is your feature request related to a problem? Please describe.**
Right now you're able to fixup a commit down into the commit below. That is, you combine the two commits and retain the message of the lower commit. However, often I find myself wanting to preserve the message of the higher commit.

**Describe the solution you'd like**
Instead of just showing a confirmation popup when fixup'ing, show a menu asking the user if they want to fixup into the commit below (current behaviour) or fixup into the commit above.