Fix memory leak.

While repository object is active and code requests commits they are stored in Map.
GHCommit.files contains huge String[]/char[]  amount of data.
The same could be applied to Milestones.

src/main/java/org/kohsuke/github/GHRepository.java

@@ -54,10 +54,10 @@ public class GHRepository {
     private boolean _private;
     private int watchers,forks,open_issues,size,network_count,subscribers_count;
     private String created_at, pushed_at;
-    private Map<Integer,GHMilestone> milestones = new HashMap<Integer, GHMilestone>();
+    private Map<Integer,GHMilestone> milestones = new WeakHashMap<Integer, GHMilestone>();
 
     private String default_branch,language;
-    private Map<String,GHCommit> commits = new HashMap<String, GHCommit>();
+    private Map<String,GHCommit> commits = new WeakHashMap<String, GHCommit>();
 
     private GHRepoPermission permissions;