dfs: Remove synchronization in BlockBasedFile#LazyChannel

As explained in 'The "Double-Checked Locking is Broken"
Declaration'[*], Java's memory model does not support double-checked
locking:

 class LazyReadableChannel {
   private ReachableChannel rc = null;

   public ReadableChannel get() {
     if (rc == null) {
       synchronized (this) {
         if (rc == null) {
           rc = new ReadableChannel();
	 }
       }
     }
     return rc;
   }
 }

With JDK 5 and newer, there is a formal memory model that ensures this
works if "rc" is volatile, but it is still not thread-safe without
that.

Fortunately, this ReadableChannelSupplier is never passed between
threads, so it does not need to be thread-safe.  Simplify by removing
the synchronization.

[*] https://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

Change-Id: I0698ee6618d734fc129dd4f63fc047c1c17c94a9
Signed-off-by: Jonathan Nieder <jrn@google.com>

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

@@ -213,7 +213,7 @@ abstract class BlockBasedFile {
 		private final DfsPackDescription desc;
 		private final PackExt ext;
 
-		private ReadableChannel rc = null;
+		private ReadableChannel rc;
 
 		LazyChannel(DfsReader ctx, DfsPackDescription desc, PackExt ext) {
 			this.ctx = ctx;
@@ -224,11 +224,7 @@ abstract class BlockBasedFile {
 		@Override
 		public ReadableChannel get() throws IOException {
 			if (rc == null) {
-				synchronized (this) {
+				rc = ctx.db.openFile(desc, ext);
-					if (rc == null) {
-						rc = ctx.db.openFile(desc, ext);
-					}
-				}
 			}
 			return rc;
 		}