demos: allow broken pipe when doing jj | head, fix opid

`jj | head` exits with non-zero code since `head` breaks the
pipe. Also, removed `--color=always` from that command as it
will shortly become unnecessary.

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

Also, the operation id recovery code stopped working. We
can use `jj debug operation` for this purpose now.

demos/demo_operation_log.sh

@@ -27,10 +27,10 @@ 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 --color=always | head"
+run_command_allow_broken_pipe "jj op log | head"
 
 comment "Let's undo that rebase operation:"
-rebase_op=$(jj --color=never op log | grep '^◉  ' | sed '2q;d' | cut -b4-15)
+rebase_op=$(jj --color=never op log --no-graph -T 'id.short(5)' --limit 1 --at-op @--)
 run_command "jj undo $rebase_op"
 
 comment "Note that only the rebase was undone, and the

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 ""
 }