phlgit_fetch: use '--prune' instead of '-p'

Prefer long form of arguments for clarity.

Test Plan:
$ ./precommit.sh
WIP: phlgitu_fixture: workers fetch and checkout at init
fixup! WIP: fetch specials in one go
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 ..