PackBitmapIndex: Move BitmapCommit to a top-level class

Move BitmapCommit from inside the PackWriterBitmapPreparer to a new
top-level class in preparation for improving the memory footprint of GC's
bitmap generation phase.

Change-Id: I4d404a5b3a34998b441d23105197f33d32d39670
Signed-off-by: Yunjie Li <yunjieli@google.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/pack/GcCommitSelectionTest.java

 
 import org.eclipse.jgit.internal.storage.file.GcTestCase;
 import org.eclipse.jgit.internal.storage.file.PackBitmapIndexBuilder;
-import org.eclipse.jgit.internal.storage.pack.PackWriterBitmapPreparer.BitmapCommit;
 import org.eclipse.jgit.junit.TestRepository.BranchBuilder;
 import org.eclipse.jgit.junit.TestRepository.CommitBuilder;
 import org.eclipse.jgit.lib.Constants;

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/BitmapCommit.java

+/*
+ * Copyright (c) 2020, Google LLC and others
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Distribution License v. 1.0 which is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+package org.eclipse.jgit.internal.storage.pack;
+
+import org.eclipse.jgit.lib.AnyObjectId;
+import org.eclipse.jgit.lib.ObjectId;
+
+/**
+ * A commit object for which a bitmap index should be built.
+ */
+public final class BitmapCommit extends ObjectId {
+	private final boolean reuseWalker;
+	private final int flags;
+
+	BitmapCommit(AnyObjectId objectId, boolean reuseWalker, int flags) {
+		super(objectId);
+		this.reuseWalker = reuseWalker;
+		this.flags = flags;
+	}
+
+	boolean isReuseWalker() {
+		return reuseWalker;
+	}
+
+	int getFlags() {
+		return flags;
+	}
+}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriter.java

 		PackWriterBitmapPreparer bitmapPreparer = new PackWriterBitmapPreparer(
 				reader, writeBitmaps, pm, stats.interestingObjects, config);
 
-		Collection<PackWriterBitmapPreparer.BitmapCommit> selectedCommits = bitmapPreparer
+		Collection<BitmapCommit> selectedCommits = bitmapPreparer
 				.selectCommits(numCommits, excludeFromBitmapSelection);
 
 		beginPhase(PackingPhase.BUILDING_BITMAPS, pm, selectedCommits.size());
 
 		BitmapWalker walker = bitmapPreparer.newBitmapWalker();
 		AnyObjectId last = null;
-		for (PackWriterBitmapPreparer.BitmapCommit cmit : selectedCommits) {
+		for (BitmapCommit cmit : selectedCommits) {
 			if (!cmit.isReuseWalker()) {
 				walker = bitmapPreparer.newBitmapWalker();
 			}

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/PackWriterBitmapPreparer.java

 				new ObjectWalk(reader), bitmapIndex, null);
 	}
 
-	/**
-	 * A commit object for which a bitmap index should be built.
-	 */
-	static final class BitmapCommit extends ObjectId {
-		private final boolean reuseWalker;
-		private final int flags;
-
-		BitmapCommit(AnyObjectId objectId, boolean reuseWalker, int flags) {
-			super(objectId);
-			this.reuseWalker = reuseWalker;
-			this.flags = flags;
-		}
-
-		boolean isReuseWalker() {
-			return reuseWalker;
-		}
-
-		int getFlags() {
-			return flags;
-		}
-	}
-
 	/**
 	 * Container for state used in the first phase of selecting commits, which
 	 * walks all of the reachable commits via the branch tips that are not