Do not update the ref hot bit when checking isIndexLoaded

DfsPackFile.isIndexLoaded() uses the DfsBlockCache.Ref.get() method
to check if the index loaded. However, using the get() method marks
a hot bit in the cache, which can cause the index to never be unloaded
and seem hotter than it really is. Add a has() method which only
checks if the value is not null and does not update the hot bit.

Change-Id: I7e9ed216f6e273e8f5d79ae573973197654419b4

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

@@ -556,5 +556,9 @@ public final class DfsBlockCache {
 				hot = true;
 			return v;
 		}
+
+		boolean has() {
+			return value != null;
+		}
 	}
 }

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

@@ -191,7 +191,7 @@ public final class DfsPackFile {
 	 */
 	public boolean isIndexLoaded() {
 		DfsBlockCache.Ref<PackIndex> idxref = index;
-		return idxref != null && idxref.get() != null;
+		return idxref != null && idxref.has();
 	}
 
 	/** @return bytes cached in memory for this pack, excluding the index. */