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/overrides.rs

@@ -139,13 +139,16 @@ impl OverrideBuilder {
     }
 
     /// Toggle whether the globs should be matched case insensitively or not.
-    /// 
+    ///
     /// When this option is changed, only globs added after the change will be affected.
     ///
     /// This is disabled by default.
     pub fn case_insensitive(
-        &mut self, yes: bool
+        &mut self,
+        yes: bool,
     ) -> Result<&mut OverrideBuilder, Error> {
+        // TODO: This should not return a `Result`. Fix this in the next semver
+        // release.
         self.builder.case_insensitive(yes)?;
         Ok(self)
     }