fix(filetype): ensure directory bufname ends w/ slash sep #40552

Problem:
`vim.filetype.match()` needs a cheap way to recognize directory buffers
without doing filesystem stat work.

Solution:
Ensure full buffer names for directories end in a trailing slash. Now
directory buffers can proceed through the normal 'filetype' path.

Note side-effects: session and ShaDa buffer-list restore behavior must
be compatible, so those + corresponding tests must be updated.

runtime/lua/nvim/dir.lua

@@ -158,8 +158,7 @@ end
 
 ---@param buf integer
 local function open_parent(buf)
-  local path = api.nvim_buf_get_name(buf)
-  -- TODO(barrettruth): simplify after #40552
+  local path = fs.normalize(api.nvim_buf_get_name(buf))
   local name = encode_name(fs.basename(path)) .. (vim.fn.isdirectory(path) == 1 and '/' or '')
   navigate(fs.dirname(path))
   vim.fn.search([[\C\m^\V]] .. vim.fn.escape(name, [[\]]) .. [[\m$]], 'cw')

runtime/lua/vim/_meta/api.gen.lua

@@ -511,6 +511,9 @@ function vim.api.nvim_buf_get_mark(buf, name) end
 
 --- Gets the full/absolute filepath of the buffer, or the buffer name for non-file buffers.
 ---
+--- If the buffer represents a directory, the name ends with a path separator,
+--- unless it was changed by `:file` or `nvim_buf_set_name()`.
+---
 --- @param buf integer Buffer id, or 0 for current buffer
 --- @return string # Buffer name
 function vim.api.nvim_buf_get_name(buf) end

runtime/lua/vim/_meta/vimfn.gen.lua

@@ -605,7 +605,9 @@ function vim.fn.bufloaded(buf) end
 ---
 --- The result is the name of a buffer.  Mostly as it is displayed
 --- by the `:ls` command, but not using special names such as
---- "[No Name]".
+--- "[No Name]".  If the buffer represents a directory, the name
+--- ends with a path separator, unless it was changed by |:file| or
+--- |nvim_buf_set_name()|.
 --- If {buf} is omitted the current buffer is used.
 --- If {buf} is a Number, that buffer number's name is given.
 --- Number zero is the alternate buffer for the current window.

runtime/lua/vim/filetype.lua

@@ -3254,6 +3254,9 @@ function M.match(args)
   end
 
   if name then
+    if name:sub(-1) == '/' then
+      return 'directory'
+    end
     name = normalize_path(name)
 
     local ok_abspath, path = pcall(vim.fs.abspath, name)