Mark: it turns out that GNU readline has a rather odd interface, only 
the index of get_history_item is 1-based, all others are 0-based. This 
is not mentioned in the documentation (neither that of the readline 
module or the GNU documentation).

I've attached a very minimal testcase for the history manipulation 
functions in the readline module that passes with GNU readline.

The tests now also pass using libedit emulation (after fixing the 
compilation issues noted by Zvezdan).

I'll file a new issue about the readline documentation, that's unrelated 
to getting libedit support into the repository.