worktree: prune duplicate entries referencing same worktree path

A fundamental restriction of linked working trees is that there must
only ever be a single worktree associated with a particular path, thus
"git worktree add" explicitly disallows creation of a new worktree at
the same location as an existing registered worktree. Nevertheless,
users can still "shoot themselves in the foot" by mucking with
administrative files in .git/worktree/<id>/. Worse, "git worktree move"
is careless[1] and allows a worktree to be moved atop a registered but
missing worktree (which can happen, for instance, if the worktree is on
removable media). For instance:

    $ 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]

Help users recover from this form of corruption by teaching "git
worktree prune" to detect when multiple worktrees are associated with
the same path.

[1]: A subsequent commit will fix "git worktree move" validation to be
     more strict.

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

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