VCS: Generalize selections preserving on model refresh

Default implementation is between O(n) (for identical models) to
O(n*m) (for disjoint models). If sort order is known, this
can be reduced to O(n+m), like in Git implementation.

Change-Id: I44662a22961311cb882601d20efa9d445f74748b
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: hjk <qthjk@ovi.com>

src/plugins/vcsbase/submitfilemodel.h

@@ -65,6 +65,8 @@ public:
     // Filter for entries contained in the filter list. Returns the
     // number of deleted entries.
     unsigned int filterFiles(const QStringList &filter);
+
+    virtual void updateSelections(SubmitFileModel *source);
 };
 
 } // namespace VcsBase