mr list: Add --sort list option
mr list: Add --sort list option

'mr list' sorts merge requests in descending order and doesn't provide an
option to list them in ascending order.

Add a --sort option to mr list to display the merge requests by
descending (desc) or ascending (asc) order.

Signed-off-by: Prarit Bhargava <fe4655370dc87a32329d591d6e8472329333dfad@redhat.com>

zaquestion/lab,zaquestion/lab