Among other things, returning an empty string as the repeat command
disables auto-repeat, which can be useful for state-changing commands.
There's one remaining refinement to this setup, which is that for parsed
script commands, it should be possible to change an option value, or add
a new option value that wasn't originally specified, then ask lldb "make
this back into a command string". That would make doing fancy things
with repeat commands easier.
That capability isn't present in the lldb_private side either, however.
So that's for a next iteration.
I haven't added this to the docs on adding commands yet. I wanted to
make sure this was an acceptable approach before I spend the time to do
that.