Finish OpenRPC tab completion
Pull in and customise the libedit tab handler from degesch, however useless it is. It might make sense to always show all continuations on the first failure to progress in a straight way. It would be good to reprint the prompt, though the cursor may be almost anywhere and the command line several lines long, so some libedit co-ordination seems necessary.
-O switch in the manual page. I don't feel comfortable making it the default as the OpenRPC standard is immature. Mention the feature in README.adoc as well.
Deleting a branch is permanent. It CANNOT be undone. Continue?