Skip pack header bytes in DfsPackFile

The 12 bytes `PACK...` header is written in PackWriter before reading
CachedPack files. In DfsPackFile#copyPackBypassCache, the header was not
skipped when the first block is not in cache.

Change-Id: Ibbe2e564d36b79922a936657f286addb1044d237
Signed-off-by: Zhen Chen <czhen@google.com>

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

@@ -499,6 +499,7 @@ public final class DfsPackFile {
 				rc.setReadAheadBytes(ctx.getOptions().getStreamPackBufferSize());
 			long position = 12;
 			long remaining = length - (12 + 20);
+			boolean packHeadSkipped = false;
 			while (0 < remaining) {
 				DfsBlock b = cache.get(key, alignToBlock(position));
 				if (b != null) {
@@ -508,6 +509,7 @@ public final class DfsPackFile {
 					position += n;
 					remaining -= n;
 					rc.position(position);
+					packHeadSkipped = true;
 					continue;
 				}
 
@@ -517,7 +519,14 @@ public final class DfsPackFile {
 					throw packfileIsTruncated();
 				else if (n > remaining)
 					n = (int) remaining;
-				out.write(buf.array(), 0, n);
+
+				if (!packHeadSkipped) {
+					// Need skip the 'PACK' header for the first read
+					out.write(buf.array(), 12, n - 12);
+					packHeadSkipped = true;
+				} else {
+					out.write(buf.array(), 0, n);
+				}
 				position += n;
 				remaining -= n;
 			}