New MR default target branch is default of target.

Created by: cirosantilli

Fixes ACCEPTING MR at http://feedback.gitlab.com/forums/176466-general/suggestions/5518180-smarter-merge-request-target-repo-and-branch-form-

One liner changer + tests ;~

Behavior before this PR: when creating a new MR, the target branch is the first one alphabetically.

Behavior after this PR: when creating a new MR, the target branch is the default of the target repo (which is currently always the current repo).

help me

Works well on interactive tests, but for the unit tests I am unable to change the current branch.

I have tried things like

@project.change_head(branch)

and

click_link("Settings")
select2(branch, from: "Default Branch")
click_button("Save changes")

and many variations without success.

Once this is settled I'll remove the test comments and prints.