Add branch text field to git/gerrit push dialog

I decided to add another field to the git/gerrit push extension dialog. I think it makes it easier to select the destination branch for the push. The initial value for the text field is equal to that of the "Push current branch to alternative branch" field (changes to that field are not carried over). I also made sure the original branch field is always disabled when the "Push to Gerrit" checkmark is checked.

Resolves #118