Fix invalid get() invocation for builtin hooks

[elsesiy]

f37e1a2#diff-8c3e398fc7377f10e5b7c253c3fa3954c027b4cab0960566faae41277f53fa1aL40 removed the get() function and cleaned up most references to it except for hooks.lua

Without this patch updating the current buffer (especially if empty) fails

Error executing vim.schedule lua callback: ...e/nvim/lazy/git-worktree.nvim/lua/git-worktree/hooks.lua:70: attempt
to call field 'get' (a nil value)
stack traceback:
        ...e/nvim/lazy/git-worktree.nvim/lua/git-worktree/hooks.lua:70: in function 'hook'
        ...e/nvim/lazy/git-worktree.nvim/lua/git-worktree/hooks.lua:43: in function 'emit'
        ...vim/lazy/git-worktree.nvim/lua/git-worktree/worktree.lua:74: in function <...vim/lazy/git-worktree.nvim/
lua/git-worktree/worktree.lua:72>