git gui: cope with duplicates in _get_recentrepo

_get_recentrepo will fail if duplicate invalid entries are present in the recentrepo config list. The previous commit fixed the 'git config' limitations in _unset_recentrepo by unsetting all config entries, however this code would fail on the second attempt to unset it. Refactor the code to pre-sort and de-duplicate the recentrepo list to avoid a potential second unset attempt. Signed-off-by: Philip Oakley <philipoakley@iee.org>