Generated code> make it harder to mark code as generated

Summary: It's now harder to accidentally mark code as generated.
Test Plan:
Tested with this diff in my sandbox; only file "D" was shown, the others
were marked as generated:

  diff --git a/A b/A
  index e69de29..780b46c 100644
  --- a/A
  +++ b/A
  @@ -0,0 +1,2 @@
  +@generated
  +Tue Jun  7 16:41:17 PDT 2011
  diff --git a/B b/B
  index e69de29..b55fe21 100644
  --- a/B
  +++ b/B
  @@ -0,0 +1,3 @@
  +/**
  + * @generated by ed
  + */
  diff --git a/C b/C
  index e69de29..e0f808a 100644
  --- a/C
  +++ b/C
  @@ -0,0 +1,3 @@
  +/**
  + * {@generated <<jonx>lol>}
  + */
  diff --git a/D b/D
  index e69de29..89e8829 100644
  --- a/D
  +++ b/D
  @@ -0,0 +1,2 @@
  +string = STDIN.readlines
  +string.include?('@generated')

Reviewed By: jungejason
Reviewers: jungejason
CC: aran, edward, jungejason
Differential Revision: 408

src/applications/differential/parser/changeset/DifferentialChangesetParser.php

@@ -540,7 +540,30 @@ class DifferentialChangesetParser {
       ipull($this->intra, 1),
       $new_corpus);
 
-    $generated = (strpos($new_corpus_block, '@'.'generated') !== false);
+
+    /**
+     * Generated code should be marked with ' @generated', '{@generated', or
+     * the block must begin with '@generated'.
+     *
+     * Any other instances of '@generated' will be ignored.
+     */
+    $generated = false;
+    $pos = strpos($new_corpus_block, '@generated');
+    if ($pos === 0) {
+      $generated = true;
+    } else {
+      $prefix_char = substr($new_corpus_block, $pos - 1, 1);
+      switch ($prefix_char) {
+        case ' ':
+        case '{':
+          $generated = true;
+          break;
+
+        default:
+          $generated = false;
+          break;
+      }
+    }
 
     $this->specialAttributes[self::ATTR_GENERATED] = $generated;
   }