As of df0b6cf (worktree: new place for "git prune --worktrees",
2015-06-29), linked worktree pruning functionality moved from
"git prune --worktrees" to "git worktree prune". Rename the
associated configuration variable accordingly.
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>