Fixes#2868, where pressing <shift-tab> then <ctrl-d> in history
completion (with > 256 items) would cause later items to disappear (and
cause a crash if you try to delete again).
Cause:
Scrolling to the bottom would fetch an additional 256 items (in addition
to the 256 that are fetched at first). Deleting causes the query to
re-run, but it only fetches the initial 256 items, so the current index
is now invalid.
Fix:
After deleting from the history category, call fetchMore until it has
enough rows populated that the current index is valid.
c6cb6ccd07