worktree: make "move" refuse to move atop missing registered worktree

"git worktree add" takes special care to avoid creating a new worktree
at a location already registered to an existing worktree even if that
worktree is missing (which can happen, for instance, if the worktree
resides on removable media). "git worktree move", however, is not so
careful when validating the destination location and will happily move
the source worktree atop the location of a missing worktree. This leads
to the anomalous situation of multiple worktrees being associated with
the same path, which is expressly forbidden by design. For example:

    $ git clone foo.git
    $ cd foo
    $ git worktree add ../bar
    $ git worktree add ../baz
    $ rm -rf ../bar
    $ git worktree move ../baz ../bar
    $ git worktree list
    .../foo beefd00f [master]
    .../bar beefd00f [bar]
    .../bar beefd00f [baz]
    $ git worktree remove ../bar
    fatal: validation failed, cannot remove working tree:
        '.../bar' does not point back to '.git/worktrees/bar'

Fix this shortcoming by enhancing "git worktree move" to perform the
same additional validation of the destination directory as done by "git
worktree add".

While at it, add a test to verify that "git worktree move" won't move a
worktree atop an existing (non-worktree) path -- a restriction which has
always been in place but was never tested.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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