redirect command stdout to stderr

Otherwise it will be interpreted as compute program protocol

doc/design/compute_special_remote_interface.mdwn

@@ -110,5 +110,5 @@ An example `git-annex-compute-foo` shell script follows:
     echo REPRODUCIBLE
 
     if [ -n "$input" ]; then
-        frobnicate --passes="$ANNEX_COMPUTE_passes" <"$input" >"$output"
+        frobnicate --passes="$ANNEX_COMPUTE_passes" -i "$input" -o "$output" >&2
     fi

doc/special_remotes/compute/git-annex-compute-imageconvert

@@ -17,5 +17,5 @@ echo "OUTPUT $2"
 read output
 
 if [ -n "$input" ]; then
-	convert "$input" "$output"
+	convert "$input" "$output" >&2
 fi