VCS[git]: Add branch combo to the checkout wizard.

Provide UI with manual refresh button in BaseCheckoutWizardPage.
Change ProcessCheckoutJob to be able to execute several steps.
Implement in git.

src/plugins/git/gitclient.h

@@ -205,6 +205,7 @@ public:
                            bool *onBranch = 0);
 
     void launchGitK(const QString &workingDirectory);
+    QStringList synchronousRepositoryBranches(const QString &repositoryURL);
 
     GitSettings  settings() const;
     void setSettings(const GitSettings &s);