ignore: speed up Gitignore::empty

This commit makes Gitignore::empty a bit faster by avoiding allocation
and manually specializing the implementation instead of routing it through
the GitignoreBuilder.

This helps improve uses of ripgrep that traverse *many* directories, and
in particular, when the use of ignores is disabled via command line
switches.

Fixes #835, Closes #836

globset/src/lib.rs

@@ -288,6 +288,14 @@ pub struct GlobSet {
 }
 
 impl GlobSet {
+    /// Create an empty `GlobSet`. An empty set matches nothing.
+    pub fn empty() -> GlobSet {
+        GlobSet {
+            len: 0,
+            strats: vec![],
+        }
+    }
+
     /// Returns true if this set is empty, and therefore matches nothing.
     pub fn is_empty(&self) -> bool {
         self.len == 0