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.

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)