Add merge squash action

[ylecuyer]

This PR adds the merge squash action to the graph. I'm waiting for your feedback before writing tests :)

[campersau]

Looks good and maybe easier for some to understand then doing it the other way around via Squash.
Is it possible to highlight the commits which are going to be merge squashed like in Squash?

Squash:

Merge Squash:

[Hirse]

What is the difference in the result between "Squash" and "Merge Squash"?

If the outcome is the same, I would be in favor of having the name, color, and icon of the button also be the same.

[ylecuyer]

Looks good and maybe easier for some to understand then doing it the other way around via Squash.
Is it possible to highlight the commits which are going to be merge squashed like in Squash?

Squash:

Merge Squash:

done

What is the difference in the result between "Squash" and "Merge Squash"?

It is the same if you select the right nodes. But I prefer to have the proposed named in the PR as this is the common wording used to describe this action, see for example the github button :

[Hirse]

So, if I understand correctly, the new "Merge Squash" does the same as the existing "Squash" on the newest (topmost) commit?

1. Should we change it to "Squash Merge" then?
2. Should we change the previous "Squash" to "Squash Merge" too then?

[campersau]

After thinking more about the UI I also think @Hirse renames suggestions would make sense. Also it might be good to unify the two squash actions even further by using the same color and icon.

Here is my suggestion:

I have picked the Squash & Merge spelling because we already have Commit & Push elsewhere.

What do you/others think?