Do not retain body in RevisionParser walk

Among other steps, this walk is used for a visibility check, so the
cumulative size of the commit content retained can be large.

Change-Id: Ia58cea7d355b569b5a6aa2ddbe459cff51180df6

java/com/google/gitiles/RevisionParser.java

diff --git a/java/com/google/gitiles/RevisionParser.java b/java/com/google/gitiles/RevisionParser.java
index 9b2d044..89311a3 100644
--- a/java/com/google/gitiles/RevisionParser.java
+++ b/java/com/google/gitiles/RevisionParser.java
@@ -108,6 +108,8 @@
       path = path.substring(1);
     }
     try (RevWalk walk = new RevWalk(repo)) {
+      walk.setRetainBody(false);
+
       Revision oldRevision = null;
 
       StringBuilder b = new StringBuilder();