ripgrep: add --ignore-file-case-insensitive

The --ignore-file-case-insensitive flag causes all
.gitignore/.rgignore/.ignore files to have their globs matched without
regard for case. Because this introduces a potentially significant
performance regression, this is always disabled by default. Users that
need case insensitive matching can enable it on a case by case basis.

Closes #1164, Closes #1170

ignore/src/walk.rs

@@ -764,6 +764,14 @@ impl WalkBuilder {
         self
     }
 
+    /// Process ignore files case insensitively
+    ///
+    /// This is disabled by default.
+    pub fn ignore_case_insensitive(&mut self, yes: bool) -> &mut WalkBuilder {
+        self.ig_builder.ignore_case_insensitive(yes);
+        self
+    }
+
     /// Set a function for sorting directory entries by their path.
     ///
     /// If a compare function is set, the resulting iterator will return all