fix(cmd): :update writes new file buffers only for real files #35939

Problem: :update should write new file buffers, but previous fix
affected special buffer types (acwrite, nofile, etc.).

Solution: Add bt_nofilename() check to only write new files for
buffers representing real filesystem paths.

runtime/doc/editing.txt

@@ -989,7 +989,8 @@ slower (but safer).
 							*:up* *:update*
 :[range]up[date][!] [++opt] [>>] [file]
 			Like ":write", but only write when the buffer has been
-			modified.
+			modified, or when the buffer represents a new file
+			that doesn't exist on disk.
 
 
 WRITING WITH MULTIPLE BUFFERS				*buffer-write*

runtime/doc/vim_diff.txt

@@ -308,6 +308,7 @@ Commands:
 - |:trust|
 - User commands can support |:command-preview| to show results as you type
 - |:write| with "++p" flag creates parent directories.
+- |:update| command writes new file buffers even when unmodified.
 
 Editor:
 - |prompt-buffer| supports multiline input/paste, undo/redo, and o/O normal

src/nvim/ex_cmds.c

@@ -1772,7 +1772,9 @@ void ex_file(exarg_T *eap)
 /// ":update".
 void ex_update(exarg_T *eap)
 {
-  if (curbufIsChanged() || (curbuf->b_ffname != NULL && !os_path_exists(curbuf->b_ffname))) {
+  if (curbufIsChanged()
+      || (!bt_nofilename(curbuf) && curbuf->b_ffname != NULL
+          && !os_path_exists(curbuf->b_ffname))) {
     do_write(eap);
   }
 }

test/functional/ex_cmds/write_spec.lua

@@ -258,4 +258,15 @@ describe(':update', function()
     eq(1, eval('g:write_pre'))
     eq(1, eval('g:write_post'))
   end)
+
+  it('does not write acwrite buffer when unchanged', function()
+    command('file remote://test')
+    command('setlocal buftype=acwrite')
+    command('let g:triggered = 0 | autocmd BufWriteCmd remote://* let g:triggered = 1')
+    command('update')
+    eq(0, eval('g:triggered'))
+    command('call setline(1, ["hello"])')
+    command('update')
+    eq(1, eval('g:triggered'))
+  end)
 end)