IDEA-130848 Push dialog initial selection & enabling policy

* remember unselected roots and store in workspace.xml;
* don't select and don't load excludedRoots;
* always enable push button for mercurial;
* load all repos for git exclude remembered;
* wait until all or one repo loaded according to sync branch settings;
* always allow push for git new branch;
* preselect git repos only from project view and ignore current editor 'selection'