Refined create branch dialog

[niik]

As me and @donokuda were looking over our dialogs we found that the merge branch dialog and the create branch dialog were in need of some design time. The merge dialog is getting its touch-up in #1126 but I forgot to open an issue about the create dialog.

Current state

The select dropdown here is pretty awful, not only in its design but also in how user-friendly it is.

Suggestion

We looked at the main flows of the create branch dialog and settled on this very streamlined approach where the user can pick between (at most) two branches to base the new one on from a vertical segmented control. We've simplified the dialog to only let you pick between basing your branch off of the current branch or the default branch (if the current branch is the default branch then we'll just show a message that the branch will be created from it and hide the selector).

We've operated under the assumption that we should switch what branch is selected by default from the current branch to the default branch. This is probably the most contentious issue and I'd love some discussion around it. Our thinking was that creating a branch that's not based off of master (in the majority of cases) is the exception rather than the norm. The counter argument to that is that it deviates from how the CLI works by default.

As for creating a branch based off another branch than the default or current our thinking is that that is such an edge-case that it's reasonable to expect users to switch to that branch first before creating their new branch. Feedback from the beta and early users will determine if we're right or wrong in that assumption.

When we did the create branch flow for Classic @SteveSmith's mocks had it so that 'master' would be selected by default and I argued against it at the time but ultimately I think he was right and I was wrong.

I've tentatively put this in M3 as I think we have room for it in there but feel free to move to M4 if you feel like it @joshaber

[joshaber]

Ha, I wrote an issue about Create Branch yesterday and then decided to look the other way 😝

I agree it's not great in its current form and that design looks great to me.

That said, I'd like to move it to M4. The current isn't great but it's functional. I think we have some bigger fish to fry before we do this.

[niik]

That said, I'd like to move it to M4. The current isn't great but it's functional. I think we have some bigger fish to fry before we do this.

👍

[iAmWillShepherd]

How would this work for alternate workflows? For instance, in my experience, I've had a dev branch that I used as a default branch.

[joshaber]

The default branch can be a non-master branch. It can be changed in the repository settings: https://help.github.com/articles/setting-the-default-branch/

[niik]

I'm gonna take a stab at this while @iAmWillShepherd finishes off #1231 and #1080