Git: Move msgRepositoryLabel from StashDialog to GitPlugin

It's being used by many dialogs.

Change-Id: I3ef484cf8b9cc56210be1de5b4f201eb2f96ef67
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/gitplugin.h

@@ -87,6 +87,7 @@ public:
 
     Gerrit::Internal::GerritPlugin *gerritPlugin() const;
     bool isCommitEditorOpen() const;
+    static QString msgRepositoryLabel(const QString &repository);
 
 public slots:
     void startCommit();