kas: Support --update argument

By default we do not update a repository if the desired refspec is
already checked out and so we do not pull any new commits that may have
been added to this refspec upstream. If the new `--update` argument is
passed on the command line then we instead pull in any new upstream
commits.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/kas.py

@@ -109,6 +109,10 @@ def setup_parser_common_args(parser):
     parser.add_argument('--force-checkout', action='store_true',
                         help='Always checkout the desired refspec of each '
                         'repository, discarding any local changes')
+    parser.add_argument('--update', action='store_true',
+                        help='Pull new upstream changes to the desired '
+                        'refspec even if it is already checked out locally')
+
 
 def kas_get_argparser():
     """