When the commandline reads ':open |', quick-completing the only offered
completion will set the commandline to ':open some_url |'. Since `open`
has `maxsplit=0`, everything after ':open' is (correctly) treated as
one argument. This means completion is opened again with 'some url '
as the pattern (note trailing whitespace), which makes the comletion
menu 'flicker' and stay open even though it was 'supposed' to quick
compelte.
This is fixed by ignoring the next completion request if we just
completed something after maxsplit (because we don't expect any more
completions after the last split).
Resolves#1519.
808a645b40