Git: Block branch refresh for actions that open a new editor

Refresh is called twice - first time when the editor is just opened, on
this call the repository can be either empty, or the active project (even
if the current file is from another repository). After the editor is
initialized, setSource is called, and the state is recovered.

But between these 2 calls, m_repository is empty, and synchronous actions
are still executed (and fail).

Fixes: QTCREATORBUG-26952
Change-Id: I1767b35f0e9f24da2c447d0b565b410742c560d9
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Robert Löhning <robert.loehning@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/git/branchview.h

@@ -97,6 +97,7 @@ private:
     BranchModel *m_model = nullptr;
     BranchFilterModel *m_filterModel = nullptr;
     Utils::FilePath m_repository;
+    bool m_blockRefresh = false;
 };
 
 class BranchViewFactory : public Core::INavigationWidgetFactory