_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>