arcyd fsck: add a '--verbose' argument

Make it easier to see what arcyd fsck is up to by adding a '--verbose'
argument.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
    exercise new argument
    $ cd ../arcyd2/
    $ arcyd fsck
    $ arcyd fsck -v
    .. lists repositories ..
    $ arcyd fsck --verbose
    .. lists repositories ..
arcyd fsck: remove legacy 'landinglog' ref

In order to clear the path for faster fetching using just refspecs as
they were intended, remove the legacy 'landinglog' ref from
repositories.

This is because fetches to refs/heads/* of refs that don't look like
branches seem to cause trouble.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh

    Setup a fake landinglog by pushing the 'poke.sh' file to the landing
    log ref. Find the ref for the 'poke.sh' file "<<POKEHASH>>"

    $ git show
    $ git cat-file -p <<COMMITHASH>>
    $ git cat-file -p <<TREEHASH>>
    $ git cat-file -p <<POKEHASH>>
    .. poke.sh contents ..
    $ git push origin <<POKEHASH>>:refs/arcyd/landinglog

    Use 'arcyd fsck' to remove the fake landinglog

    $ cd ../arcyd2/
    $ arcyd fsck --remote
    .. reports landinglog ..
    $ arcyd fsck --remote --fix
    .. removes landinglog ..
    $ arcyd fsck --remote
    .. no errors ..
    $ cd ../dev
    $ git ls-remote
    .. no landinglog ..
arcyd fsck: check specific list of repos

Make it possible to check a partial list of repositories by adding a new
parameter to support this.

This is useful if a single repo needs to be checked or to enable a
workflow where errors are listed first and fixed individually.

Test Plan:
$ ./precommit.sh

Make sure list of repos checked is as expected
$ ./testbed/arcyd/test_shell.sh
    $ cd ../arcyd2
    $ arcyd fsck -v
    .. lists repos checked ..
    $ arcyd fsck -v --repos <<valid repo>>
    .. lists single repo checked ..
    $ arcyd fsck -v --repos <<valid repo 1>> <<valid repo 2>>
    .. lists both repos checked ..
    $ arcyd fsck -v --repos
    .. error message ..
    $ arcyd fsck -v --repos <<INVALID repo>>
    .. error message ..