Vcs: allow settings to read legacy data

As advised with Tobias, having a "read legacy" annex
function to readSettings() is the simplest solution for
backward compatibility with old VCS settings.
readLegacySettings() is useful when refactoring a Vcs
settings class to make it inherit VcsBaseClientSettings.

Change-Id: I96e3f67921f607bf010688d023098114e0e91035
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/vcsbase/vcsbaseclientsettings.cpp

@@ -254,6 +254,8 @@ void VcsBaseClientSettings::readSettings(const QSettings *settings)
             break;
         }
     }
+
+    this->readLegacySettings(settings);
 }
 
 bool VcsBaseClientSettings::equals(const VcsBaseClientSettings &rhs) const
@@ -381,4 +383,9 @@ QVariant VcsBaseClientSettings::keyDefaultValue(const QString &key) const
     return QVariant(valueType(key));
 }
 
+void VcsBaseClientSettings::readLegacySettings(const QSettings *settings)
+{
+    Q_UNUSED(settings);
+}
+
 } // namespace VcsBase