RFR: JDK-8185426: Jshell crashing on autocompletion

[Jan Lahoda]

I'd like to ask for a review of a fix for bug:
https://bugs.openjdk.java.net/browse/JDK-8185426

Proposed webrev:
http://cr.openjdk.java.net/~jlahoda/8185426/webrev.00/

The problem here is that: jshell has a stateful completion, pressing 
<tab> several times will produce different outputs. The state of the 
completion is currently reset when the user performs action different 
than "COMPLETE" as determined by KeyMap. Sadly, even if the action is 
COMPLETE, JLine may (if there are further characters in the input) 
decide it won't perform the complete action, rather print the tab 
character. See:
http://hg.openjdk.java.net/jdk10/jdk10/jdk/file/0861f2298fd7/src/jdk.internal.le/share/classes/jdk/internal/jline/console/ConsoleReader.java#l2669

This then can lead to the problem, as the completion may be using the 
stale state.

So the patch is trying to ensure the completion state is reset if 
complete is not called: the state is reset always, and the complete 
method will restore it if needed.

How does this look?

Thanks,
     Jan