Add MutableObjectId.copyFrom(AnyObjectId)

This simplifies the PackIndex code, which is trying to quickly copy
an existing ObjectId into a MutableObjectId.  Rather than having
the PackIndex violate the ObjectId's internals, expose a copy from
function similar to the other ones for copying from raw byte arrays
or hex formatted strings.

Change-Id: I142635cbece54af2ab83c58477961ce925dc8255
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/lib/MutableObjectId.java

@@ -71,11 +71,7 @@ public class MutableObjectId extends AnyObjectId {
 	 *            original entry, to copy id from
 	 */
 	MutableObjectId(MutableObjectId src) {
-		this.w1 = src.w1;
-		this.w2 = src.w2;
-		this.w3 = src.w3;
-		this.w4 = src.w4;
-		this.w5 = src.w5;
+		fromObjectId(src);
 	}
 
 	/** Make this id match {@link ObjectId#zeroId()}. */
@@ -87,6 +83,20 @@ public class MutableObjectId extends AnyObjectId {
 		w5 = 0;
 	}
 
+	/**
+	 * Copy an ObjectId into this mutable buffer.
+	 *
+	 * @param src
+	 *            the source id to copy from.
+	 */
+	public void fromObjectId(AnyObjectId src) {
+		this.w1 = src.w1;
+		this.w2 = src.w2;
+		this.w3 = src.w3;
+		this.w4 = src.w4;
+		this.w5 = src.w5;
+	}
+
 	/**
 	 * Convert an ObjectId from raw binary representation.
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/lib/PackIndex.java

@@ -281,11 +281,7 @@ public abstract class PackIndex implements Iterable<PackIndex.MutableEntry> {
 		public MutableEntry cloneEntry() {
 			final MutableEntry r = new MutableEntry();
 			ensureId();
-			r.idBuffer.w1 = idBuffer.w1;
-			r.idBuffer.w2 = idBuffer.w2;
-			r.idBuffer.w3 = idBuffer.w3;
-			r.idBuffer.w4 = idBuffer.w4;
-			r.idBuffer.w5 = idBuffer.w5;
+			r.idBuffer.fromObjectId(idBuffer);
 			r.offset = offset;
 			return r;
 		}