Add options to `Update from upstream/*` function

### The feature request

Currently `Branch -> Update from upstream/*` always `Merge`s.
I'd like the option for it to instead do `Rebase and Merge`.

Or ideally, an option to disable merge commits.

### Reasoning

Our repository only has `Allow rebase merging` enabled. (We value commit history)

Meaning we really do not want users to have merge commits, and as Github Desktop really loves those, it's an unnecessary pita when less tech savvy contributors need to update their Pull Requests.