Add a workaround for using autochdir

Related to justinmk#19

Allow the :cd on autocmd emulation of autochdir to work with dirvish.
Describe this workaround for autochdir-like behavior in the help.

This change might be a first step towards autochdir, but it's not enough
yet.

The autocmd breaks dirvish when we try to force an alternate file
because we're :cd-ing while dirvish is trying to do stuff and we end up
with an empty buffername. This change tracks when we're in set_altbuf so
the autocmd can be disabled during that region (using noau didn't work
and replacing set_altbuf with keepalt everywhere didn't work either).

Ideally, we detect if autochdir is enabled and re-enable it when we're
clear of set_altbuf, but it doesn't work yet.

autoload/dirvish.vim

@@ -2,6 +2,9 @@ let s:sep = (&shell =~? 'cmd.exe') ? '\' : '/'
 let s:noswapfile = (2 == exists(':noswapfile')) ? 'noswapfile' : ''
 let s:noau       = 'silent noautocmd keepjumps'
 
+" Can autochdir when <= 0, should set autochdir when < 0.
+let s:dirvish_autochdir_blocked_level = 0 - &autochdir
+
 function! s:msg_error(msg) abort
   redraw | echohl ErrorMsg | echomsg 'dirvish:' a:msg | echohl None
 endfunction
@@ -219,11 +222,21 @@ function! s:open_selected(split_cmd, bg, line1, line2) abort
 endfunction
 
 function! s:set_altbuf(bnr) abort
+  set noautochdir
+  let s:dirvish_autochdir_blocked_level += 1
+
   let curbuf = bufnr('%')
   call s:try_visit(a:bnr)
   let noau = bufloaded(curbuf) ? 'noau' : ''
   " Return to the current buffer.
   execute 'silent keepjumps' noau s:noswapfile 'buffer' curbuf
+
+  let s:dirvish_autochdir_blocked_level -= 1
+  if s:should_autochdir()
+    " TODO: Using autochdir still breaks other things (buffername is sometimes
+    " empty).
+    "set autochdir
+  endif
 endfunction
 
 function! s:try_visit(bnr) abort
@@ -375,9 +388,17 @@ function! s:buf_isvalid(bnr) abort
   return bufexists(a:bnr) && !isdirectory(s:sl(bufname(a:bnr)))
 endfunction
 
+function! s:should_autochdir() abort
+  return s:dirvish_autochdir_blocked_level == -1
+endfunction
+
+function! dirvish#can_autochdir() abort
+  return s:dirvish_autochdir_blocked_level <= 0
+endfunction
+
 function! dirvish#open(...) range abort
   if &autochdir
-    call s:msg_error("'autochdir' is not supported")
+    call s:msg_error("'autochdir' is not supported. See help for workaround.")
     return
   endif
   if !&hidden && &modified

doc/dirvish.txt

@@ -168,5 +168,13 @@ Check your 'isfname' setting. On Windows (where filepaths may contain
 backslashes "\") |fnamemodify()| may produce nonsense if 'isfname' does not
 contain the aforementioned unusual characters.
 
+Is there a way to use 'autochdir'?~
+Since autochdir is not supported, you can use this workaround: >
+    " 'autochdir' alternative to specify exceptions.
+    " Switch to the directory of the current file, unless it's a help or conflicts with dirvish.
+    autocmd BufEnter * if dirvish#can_autochdir() && match(['help', 'dirvish'], printf("\<%s\>", &ft)) < 0 && filereadable(expand("%")) | silent! cd %:p:h | endif
+<
+Alternatively, you can use |BufReadPost| instead of |BufEnter| to change directories only on load.
+
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  vim:tw=78:ts=8:ft=help:norl: