Switch to commonmark 0.5.0

Reasons to switch:

  * Supports the CommonMark[1] "standard"
  * Fewer dependencies
  * BSD 2-clause license
  * Fast (claims 10-20 times faster than pegdown)
  * Cleaner extension API than Pegdown
  * No timeout limit required

Testing with Documentation/markdown.md:

        | cold vm | hot vm |
        +---------+--------|
  prior |   2.29s | 279ms  |
   this |   1.64s | 139ms  |

Testing with Chromium's botmap.md:

        | cold vm | hot vm |
        +---------+--------|
  prior |   3.47s | 946ms  |
   this |   1.83s | 257ms  |

Changes in Markdown from Pegdown:

  * No <<double angle>> quotes (confused with HTML).
  * No table captions (no [caption after table]).
  * Table columns are same width (no multi-column spans).
  * Smart quotes can no longer be disabled by \'.
  * Horizontal rule should be at least 3 -, e.g. ---.
    (However GitHub Flavor -- is still accepted.)

[1] http://commonmark.org

Change-Id: Ief189d0226901a284d9b86d9798310786ae1e983

gitiles-servlet/BUCK

diff --git a/gitiles-servlet/BUCK b/gitiles-servlet/BUCK
index 81c06e2..d66d8a2 100644
--- a/gitiles-servlet/BUCK
+++ b/gitiles-servlet/BUCK
@@ -3,12 +3,14 @@
 DEPS = [
   '//blame-cache:lib',
   '//lib:commons-lang',
-  '//lib:grappa',
   '//lib:gson',
   '//lib:guava',
   '//lib:joda-time',
   '//lib:jsr305',
-  '//lib:pegdown',
+  '//lib:commonmark',
+  '//lib:cm-autolink',
+  '//lib:gfm-tables',
+  '//lib:gfm-strikethrough',
   '//lib:prettify',
   '//lib/jgit:jgit',
   '//lib/jgit:jgit-servlet',