Add popup if upstream branch is not set when pushing

[WizardOhio24]

Closes #492

Adds a popup on push with the list of possible remote branches if no remote branch is set. At the bottom of the list is the name of the branch which may exist in remote repo but which can be pushed (-u) and added if it is not there. This allows the user to select one of the remote branches which are connected to their repo or push their own branch name.

Would it be better to not list all the remote branches? i.e only list origin/<branch_name>, <some_remote>/<branch_name>, or do it the way it is right now, with all the branches?
Perhaps a drop down list, where origin and <some_remote> could be selected to push to them with the branch name, or they could be dropped down to selected a specific branch.

And should the user be able to set the branch upstream by pressing, e.g U on in the status tab to bring up the upstreams? I think a dedicated branch tab for dealing with local/remote branches would be better, but would be happy to add a Set upstream [U] button.

Any suggestions on how to handle this would be appreciated.

[extrawurst]

please refer to #491 for the details, the point is to allow selecting the remote not the branchname - a list of all available remotes is sufficient to fix #491

[stale]

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