This PR introduces a new sub-crate, `ignore`, which primarily provides a
fast recursive directory iterator that respects ignore files like
gitignore and other configurable filtering rules based on globs or even
file types.
This results in a substantial source of complexity moved out of ripgrep's
core and into a reusable component that others can now (hopefully)
benefit from.
While much of the ignore code carried over from ripgrep's core, a
substantial portion of it was rewritten with the following goals in
mind:
1. Reuse matchers built from gitignore files across directory iteration.
2. Design the matcher data structure to be amenable for parallelizing
directory iteration. (Indeed, writing the parallel iterator is the
next step.)
Fixes#9, #44, #45
In particular, if we had an inner literal and were doing a case insensitive
search, then the literals are dropped because we previously only allowed
a single inner literal to have an effect. Now we allow alternations of
inner literals, but still don't quite take full advantage.
