This flag is similar to what's found in grep: it will suppress all error
messages, such as those shown when a particular file couldn't be read.
Closes#149
This adds a new walk type in the `ignore` crate, `WalkParallel`, which
provides a way for recursively iterating over a set of paths in parallel
while respecting various ignore rules.
The API is a bit strange, as a closure producing a closure isn't
something one often sees, but it does seem to work well.
This also allowed us to simplify much of the worker logic in ripgrep
proper, where MultiWorker is now gone.
