The 'git backfill' command uses the path-walk API in a critical way: it
uses the objects output from the command to find the batches of missing
objects that should be requested from the server. Unlike 'git
pack-objects', we cannot fall back to another mechanism.
The previous change added the path_walk_filter_compatible() method that
we can reuse here. Use it during argument validation in cmd_backfill().
Signed-off-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>