resolves #565 Add read-only option for hgrc

Created originally on Bitbucket by colin-hurley (Colin Hurley)

Was already merged in Bitbucket before import, marked as merged by the import user

This changeset adds the "Do not modify the repository hgrc file" setting to the general preferences page. The setting is disabled by default, so the plugin behavior does not change without explicit user action. When the setting is enabled, no modifications will be made to hgrc files when a new default repository is selected (i.e. those changes will be persisted only in the Eclipse preferences).