Add detection for Hack files with ".php" file extension

Based on top of PR#1447. Adds a simple heuristic check for Hack files vs PHP files (`<?hh` vs other `<?`).

Tested by verifying that the Hack example site was detected as 100% Hack and that Laravel was detected as 100% PHP. (Without the heuristic, Laravel gets detected as about 50% Hack, just by randomness in the classifier since PHP and Hack are very hard to distinguish unless you actually parse the file and look for specific language features.)

lib/linguist/heuristics.rb

@@ -25,6 +25,9 @@ module Linguist
         if languages.all? { |l| ["Common Lisp", "OpenCL"].include?(l) }
           result = disambiguate_cl(data, languages)
         end
+        if languages.all? { |l| ["Hack", "PHP"].include?(l) }
+          result = disambiguate_hack(data, languages)
+        end
         return result
       end
     end
@@ -88,6 +91,13 @@ module Linguist
       matches
     end
 
+    def self.disambiguate_hack(data, languages)
+      matches = []
+      matches << Language["Hack"] if data.include?("<?hh")
+      matches << Language["PHP"] if /<?[^h]/.match(data)
+      matches
+    end
+
     def self.active?
       !!ACTIVE
     end

lib/linguist/languages.yml

@@ -986,6 +986,7 @@ Hack:
   ace_mode: php
   extensions:
   - .hh
+  - .php
 
 Haml:
   group: HTML

samples/Hack/funs.php

@@ -0,0 +1,32 @@
+<?hh
+/**
+ * Copyright (c) 2014, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ */
+
+/**
+ * This file contains a bunch of php stubs for functions that have been added
+ * to hhvm (though aren't in a release yet). These are important because the
+ * Hack typechecker can understand them
+ */
+
+class InvariantViolationException extends Exception {}
+
+function invariant(mixed $test, string $message): void {
+  if (!$test) {
+    invariant_violation($message);
+  }
+}
+
+function invariant_violation(string $message): void {
+  throw new InvariantViolationException($message);
+}
+
+function class_meth(string $class, string $method) {
+  return array($class, $method);
+}