RFR: JDK-8178821: jshell tool: ctrl-down does nothing in current context

[Jan Lahoda]

Bug:
https://bugs.openjdk.java.net/browse/JDK-8178821

Webrev:
http://cr.openjdk.java.net/~jlahoda/8178821/webrev.00/

The problem here is as follows: for JShell and jjs, we wrap the class 
that provides input history (jline's MemoryHistory) with our own custom 
version, that has an extra feature to simplify editing of code snippets 
(code snippet may be a multi-line method, for example).

For this feature, there are two actions: go to the previous/next code 
snippet. These actions are implemented using MemoryHistory.index() 
[which returns the index of the currently selected history entry] and 
.size() [which returns the number of history entries].

The MemoryHistory has a limit on number of entries in it (500 by 
default). So, the .size() will be 500 at most. But, when the oldest 
entries are removed, the index() of the retained entries will not 
change, and so index() may go beyond 500. Also, as a consequence, the 
first retained entry does not necessarily have index() == 0.

The two effects this has are:
-Ctrl-down sometimes does not work as the current nextSnippet method 
won't work if index() [of the current entry] >= size()
-Ctrl-up may crash with an exception if inspecting entries that have 
been removed, as Ctrl-up currently runs as long as index() >= 0

The proposed patch is to reuse existing previous()/next() methods, which 
select previous/next history entry, and check the entry index against 
size correctly.

Thanks,
     Jan