args: allow to use multiple `--refs` arguments

Example: `git filter-repo --refs head1 --refs head2`

Signed-off-by: Nikita Sliusarev <nsl@nebius.com>

git-filter-repo

@@ -1990,7 +1990,7 @@ EXAMPLES
     #     commit has an excluded ancestor as a parent we have no way of
     #     knowing what it is an ancestor of without doing a special
     #     full-graph walk.
-    misc.add_argument('--refs', nargs='+',
+    misc.add_argument('--refs', action='append',
         help=_("Limit history rewriting to the specified refs.  Implies "
                "--partial.  In addition to the normal caveats of --partial "
                "(mixing old and new history, no automatic remapping of "