* ivy.el (ivy--reset-state): Don't push preselect onto collection for
:dynamic-collection.
(ivy-recompute-index-swiper-async): New defun. It's useful for
re-anchoring on collections produced async processes. The major
difference from `ivy-recompute-index-swiper' is using `equal' instead of `eq'.
* counsel.el (counsel--async-sentinel): Add index recomputing logic.
When `ivy--old-cands' are null, recompute the index according to
:preselect, otherwise try `ivy--recompute-index'.
(counsel-grep): New command. Very similar to `swiper', except calls an
external process for each key update. Should be much faster for very
large files, both for startup and for matching. For smaller files, it's
less convenient.
(counsel-grep-function): New defun.
(counsel-grep-action): New defun.