rebase: select remote tracking branch as default

[benmosher]

I often fetch + rebase against origin/master + push.

Assuming this is a common workflow, I thought it would be useful to auto-select the tracking remote branch in the rebase on top of... => Rebase on top of branch list.

Possible additional option, if this is very common: add as a third option next to Rebase on top of branch and Rebase on top of ref: Rebase on top of remote-tracking branch, and auto-select it if it was the last option used. Then this workflow is reduced to r + Return on the rebase screen.

Just a thought. Curious if others find this useful.

Thanks! :smile: