vim:autocmd: Do not mark stdin as modified

Set `nomodified` after reading from standard input. This way the buffer
can be closed without error message.

This will **not** disable modified warnings completely. If the input is
modified after read, vim will reset `modified` (As it should).

.config/vim/vimrc.d/80-autocommands.vim

@@ -62,3 +62,9 @@ augroup highlight_current_word
 	au CursorHold * call HighlightCurrentWord()
 	au CursorMoved * match
 augroup END
+
+" Do not mark input from stdin as modified
+augroup stdin_not_modified
+	au!
+	au StdinReadPost * set nomodified
+augroup END