Preserve and clear the saved current line properly

[daxian-dbw]

Fix #1257

Major changes:

1. Preserve the saved current line and the history index while in a series of history commands.
   So when switching between different history command, such as from F8/Shift+F8 (HistorySearchBackward and HistorySearchForward) to UpArrow/DownArrow (recall history) or from Ctrl+r/Ctrl+s to UpArrow/DownArrow, the history command can continue to work as expected.
2. Clear the saved current line when we are out of a series of history commands.
   So the saved line will not leak to another unrelated history search.

[daxian-dbw]

The newHistoryIndex == _history.Count is removed because that was already checked above:

if (newHistoryIndex < 0 || newHistoryIndex >= _history.Count)
{
    break;
}

[msftrncs]

I added to #1257 some minor points. While the changes to RevertLine() seem to prevent the exception, its not the only vector to the source of the issue.

I'm wondering if SaveCurrentLine() needs to rely on something other than a line having not already having been saved ... such as _searchHistoryCommandCount == 0, to allow it to overwrite the previously saved line.

[daxian-dbw]

Thanks for the additional details. You are right, it’s not just RevertLine, but a more general issue. Basically the saved line is not properly cleared after any history operation.
I will think a bit more and update the fix.

[daxian-dbw]

@msftrncs I think the PR is ready for another review :) Happy new year!

[msftrncs]

All LGTM.

[daxian-dbw]

Thanks @msftrncs!