rc grep: include stderr in grep buffer

Running an invalid command like "grep -abc" shows no output at all.
Let's give better feedback by showing the error message from grep.
We used to do this until an unrelated change, bd5955c73 (grep: remove
eventual \r, 2013-02-13).

rc/tools/grep.kak

@@ -25,7 +25,7 @@ define-command -params .. -docstring %{
 
      output=$(mktemp -d "${TMPDIR:-/tmp}"/kak-grep.XXXXXXXX)/fifo
      mkfifo ${output}
-     ( ${kak_opt_grepcmd} "$@" | tr -d '\r' > ${output} 2>&1 & ) > /dev/null 2>&1 < /dev/null
+     ( ${kak_opt_grepcmd} "$@" 2>&1 | tr -d '\r' > ${output} 2>&1 & ) > /dev/null 2>&1 < /dev/null
 
      printf %s\\n "evaluate-commands -try-client '$kak_opt_toolsclient' %{
                edit! -fifo ${output} *grep*