fix(restart): missing 'filetype' after session load #41975

Problem:
A buffer loaded (`bufload()`) from a (`BufAdd nested=false`) autocmd
does not trigger `BufReadPost` (which does filetype-detection). Then
'filetype' is never set.

A session file does `:badd` on all buffers:

    :badd +0 a.md
    :badd +0 b.md
    :edit a.md

If a mystery plugin(?) loads the buffers via `bufload()` from the
`nested=false BufAdd` autocmd triggered by those `:badd` calls, filetype
detection never happens.

Solution:
When `BufReadPost` was skipped and 'filetype' is empty, retry the
`filetypedetect` augroup.

`do_write` uses a similar approach:
https://github.com/neovim/neovim/blob/9047ee1460ede5aa8b0f0807c15bb554f5dcb8f1/src/nvim/ex_cmds.c#L1964-L1967

Note: Intentionally not using ":filetype detect".
- It silently re-enables detection when the user turned it off.
- It runs `do_modelines` again (redundant).
- It loses the `<afile>` name.

0 files changed, 0 insertions, 0 deletions