Proposed changes to empty command handling in JDB

[Jakob Cornell]

> I'm a bit dubious about changing jdb to have blank lines repeat like gdb. Hard to know if users of jdb expect that behavior and also it feels like basic history navigation would be better.

Well that would be addressed by our plan to leave the functionality off by default; I believe enable/disable commands are the currently preferred way, so that I can then specify something like `repeat' in my jdbrc file to enable it.

In my view line editing/history would make JDB much better, but the difference between one keystroke (Enter) per repetition with auto-repeat and two (Up, Enter) with history navigation would be noticeable when quickly stepping through a few lines of code.  Also I'd note that PDB and perhaps other debuggers already replicate GDB's auto-repeat behavior, so at least for developers with language experience beyond Java I would expect this to be a welcome change.