demos: redact one of the conflict outputs

I would prefer to shorten it instead of fully redacting, but I ran into
#4239

demos/demo_juggle_conflicts.sh

@@ -25,7 +25,7 @@ run_command "jj diff -r second"
 run_command "jj diff -r third"
 
 comment "Let's reorder the second and third commits:"
-run_command "jj rebase -s third -d first"
+run_command_output_redacted "jj rebase -s third -d first"
 run_command "jj rebase -s second -d third"
 run_command "jj log"
 comment "The commit labeled \"third\" has a conflict, as expected. What's more

demos/helpers.sh

@@ -20,7 +20,7 @@ run_command_output_redacted() {
   echo "\$ $@"
   # `bash` often resets $COLUMNS, so we also
   # allow $RUN_COMMAND_COLUMNS
-  eval "$@" > /dev/null
+  eval "$@" > /dev/null 2>&1
   echo -e "\033[0;90m... (output redacted) ...\033[0m"
 }