Improves performance of glob exclusions by avoiding extra glob IO

- IO is not necessary for exclusions because they exclude file paths that have already been discovered.  Using `file.match` / `minimatch` is sufficient.
- adds optional `excludeFn` callback to `processPatterns`. If provided, it will be called instead of `fn` for exclusion patterns.

lib/grunt/file.js

@@ -38,7 +38,8 @@ file.setBase = function() {
 
 // Process specified wildcard glob patterns or filenames against a
 // callback, excluding and uniquing files in the result set.
-var processPatterns = function(patterns, fn) {
+var processPatterns = function(patterns, fn, exclusionFn) {
+  if (typeof exclusionFn === 'undefined') { exclusionFn = fn; }
   // Filepaths to return.
   var result = [];
   // Iterate over flattened patterns array.
@@ -48,7 +49,7 @@ var processPatterns = function(patterns, fn) {
     // If the pattern is an exclusion, remove the !
     if (exclusion) { pattern = pattern.slice(1); }
     // Find all matching files for this pattern.
-    var matches = fn(pattern);
+    var matches = (exclusion ? exclusionFn : fn)(pattern, result);
     if (exclusion) {
       // If an exclusion, remove matching files.
       result = grunt.util._.difference(result, matches);
@@ -104,6 +105,8 @@ file.expand = function() {
   var matches = processPatterns(patterns, function(pattern) {
     // Find all matching files for this pattern.
     return file.glob.sync(pattern, globOptions);
+  }, function(pattern, resultsThusFar) {
+    return file.match(options, [pattern], resultsThusFar);
   });
   // Filter result set?
   if (options.filter) {