Add a DfsPackFile method to get the number of cached bytes

The counter is actually stored in the DfsPackKey so it can be
manipulated by the cache.

Change-Id: I10cee76c92d65c68d1aa1a9dd0c4fd7173c4cede

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsBlockCache.java

 		DfsBlockCache oc = cache;
 		cache = nc;
 
-		if (oc != null && oc.readAheadService != null)
-			oc.readAheadService.shutdown();
+		if (oc != null) {
+			if (oc.readAheadService != null)
+				oc.readAheadService.shutdown();
+			for (DfsPackFile pack : oc.getPackFiles())
+				pack.key.cachedSize.set(0);
+		}
 	}
 
 	/** @return the currently active DfsBlockCache. */
 				e2 = table.get(slot);
 			}
 
+			key.cachedSize.addAndGet(v.size());
 			Ref<DfsBlock> ref = new Ref<DfsBlock>(key, position, v.size(), v);
 			ref.hot = true;
 			for (;;) {
 				dead.next = null;
 				dead.value = null;
 				live -= dead.size;
+				dead.pack.cachedSize.addAndGet(-dead.size);
 				statEvict++;
 			} while (maxBytes < live);
 			clockHand = prev;
 				}
 			}
 
+			key.cachedSize.addAndGet(size);
 			ref = new Ref<T>(key, pos, size, v);
 			ref.hot = true;
 			for (;;) {

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsPackFile.java

 		return packDesc;
 	}
 
+	/** @return bytes cached in memory for this pack, excluding the index. */
+	public long getCachedSize() {
+		return key.cachedSize.get();
+	}
+
 	private String getPackName() {
 		return packDesc.getPackName();
 	}

org.eclipse.jgit/src/org/eclipse/jgit/storage/dfs/DfsPackKey.java

 
 package org.eclipse.jgit.storage.dfs;
 
+import java.util.concurrent.atomic.AtomicLong;
+
 final class DfsPackKey {
 	final int hash;
 
+	final AtomicLong cachedSize;
+
 	DfsPackKey() {
 		// Multiply by 31 here so we can more directly combine with another
 		// value without doing the multiply there.
 		//
 		hash = System.identityHashCode(this) * 31;
+		cachedSize = new AtomicLong();
 	}
 }