Add '--remote' arg to 'arcyd fsck'

It's useful to be able to check that remote resources are still
accessible without going through the snoop url and cache - they may not
be updated in the event that a repository has gone away.

Start here with checking that remote git repositories are accessible.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd2/
    $ rm -rf ../origin
    $ arcyd fsck
    $ arcyd fsck --remote
    .. error reported as expected ..
    $ arcyd fsck --remote --fix
    .. error reported as expected ..