hijack_directories Not Respected When Not `open_on_setup`

[alex-courtis]

c5536db #1618 broke this behaviour.

See #1660 (reply in thread)

[alex-courtis]

This is messy: the code and the configuration.

I'm tempted to add new configuration and deprecate the old:

on_start = {
    open = {
      always = false,
      directory = true,
      unnamed = false,
      empty = false,
      file = true,
      ignore_filetype = {
        "gitcommit"
      },
    },
    focus_tree = {
      always = true,
      unnamed = false,
      empty = false,
      file = false,
    },
  }

unnamed is a buffer with empty bufname().

empty is a buffer with a name but filereadable(bufname()) 0. Needs a better name.

The new configuration could go in an alternate nvim-tree.on_enter to be used if the user has specified on_start.

[alex-courtis]

Unfortunately this change has been reverted due to many regressions: #1668

A new mechanism to control startup behaviour is under way: #1669