Add missing +/-/' ' on diff views

The diff formatter forgot to include the prefix character at the
start of each line in the region. Add the prefix before the rest
of the line text.

Bug: http://code.google.com/p/gitiles/issues/detail?id=15
Change-Id: If8e45c8c2ae2c2b592addd1dee94c2216b2388a5

gitiles-servlet/src/main/java/com/google/gitiles/HtmlDiffFormatter.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/HtmlDiffFormatter.java b/gitiles-servlet/src/main/java/com/google/gitiles/HtmlDiffFormatter.java
index 63b1eb8..993197d 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/HtmlDiffFormatter.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/HtmlDiffFormatter.java
@@ -122,6 +122,7 @@
         out.write(LINE_CHANGE_BEGIN);
         break;
     }
+    out.write(prefix);
     out.write(StringEscapeUtils.escapeHtml4(text.getString(cur)).getBytes(Charsets.UTF_8));
     out.write(LINE_END);
   }