demos: allow broken pipe when doing jj | head

`jj | head` exits with non-zero code since `head` breaks the pipe.

Previosly, this caused the script to stop since it's run with
`set -o pipefail`.

demos/demo_operation_log.sh

@@ -27,7 +27,7 @@ comment "The repo now looks like this:"
 run_command "jj log"
 comment "The most recent portion of the operation log
 is:"
-run_command "jj op log | head"
+run_command_allow_broken_pipe "jj op log | head"
 
 comment "Let's undo that rebase operation:"
 rebase_op=$(jj debug operation --display id --at-op @-- | head --bytes 8)

demos/helpers.sh

@@ -14,6 +14,21 @@ run_command() {
   eval "$@"
 }
 
+run_command_allow_broken_pipe() {
+  run_command "$@" || {
+    EXITCODE="$?"
+    case $EXITCODE in
+    3)
+      # `jj` exits with error coded 3 on broken pipe,
+      # which can happen simply because of running
+      # `jj|head`.  
+      return 0;;
+    *)
+      return $EXITCODE;;
+    esac
+  }
+}
+
 blank() {
   echo ""
 }