* Remove "skip popup" related logic
* Fix matches not getting cleared
Before this commit we don't clear the matches when the text
under the cursor is no longer suitable for completion. This
causes e.g. removing everything using <BS> does not close
the completion menu.
* Allow closing popup menu
Closing popup menu calls completion itself, making it
impossible to be successful.
We used to have a "skip popup" logic to fix this. It was
removed 2 commits ago for being too confusing.
To fix this a check is adapted to make sure that completion
is only called when the completion context has changed.
Since closing popup menu does not change the completion
context, it no longer calls completion, and therefore can
succeed.
Previously this check lets us not complete when we move to a
different line we are in insert mode, through for example
<CR> or <BS>, but actually we are not afraid of this. When
we enter a newline, we don't complete anyway since the
refresh pattern is not matched, and when we backspace to a
previous line, and the line happens to end with something
that will trigger a completion, why do we not want to
trigger it?
* Also compare completion base to determine context change
It is possible that in some situation changing the text under the
cursor does not change the position of the cursor, so we also
compare the completion base to determine whether the context has
changed.
