diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriterTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriterTest.java
index 9f65ee2..7130d59 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriterTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriterTest.java
@@ -10,6 +10,7 @@
 
 package org.eclipse.jgit.internal.storage.commitgraph;
 
+import static java.util.stream.Collectors.toList;
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsInAnyOrder;
 import static org.junit.Assert.assertArrayEquals;
@@ -19,8 +20,12 @@
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
+import java.nio.ByteBuffer;
+import java.nio.charset.StandardCharsets;
 import java.util.Collections;
 import java.util.HashSet;
+import java.util.List;
+import java.util.Optional;
 import java.util.Set;
 
 import org.eclipse.jgit.dircache.DirCacheEntry;
@@ -413,6 +418,25 @@ public void testReuseBloomFilters() throws Exception {
 				"119,69,63,-8,0,"));
 	}
 
+	@Test
+	public void testPathDiffCalculator_skipUnchangedTree() throws Exception {
+		RevCommit root = tr.commit(tr.tree(
+				tr.file("d/sd1/f1", tr.blob("f1")),
+				tr.file("d/sd2/f2", tr.blob("f2"))));
+		RevCommit tip = tr.commit(tr.tree(
+				tr.file("d/sd1/f1", tr.blob("f1")),
+				tr.file("d/sd2/f2", tr.blob("f2B"))), root);
+		CommitGraphWriter.PathDiffCalculator c = new CommitGraphWriter.PathDiffCalculator();
+
+		Optional<HashSet<ByteBuffer>> byteBuffers = c.changedPaths(walk.getObjectReader(), tip);
+
+		assertTrue(byteBuffers.isPresent());
+		List<String> asString = byteBuffers.get().stream()
+				.map(b -> StandardCharsets.UTF_8.decode(b).toString())
+				.collect(toList());
+		assertThat(asString, containsInAnyOrder("d", "d/sd2", "d/sd2/f2"));
+	}
+
 	RevCommit commit(RevCommit... parents) throws Exception {
 		return tr.commit(parents);
 	}
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java
index 0d9815e..37a0de8 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/commitgraph/CommitGraphWriter.java
@@ -71,6 +71,9 @@ public class CommitGraphWriter {
 
 	private static final int MAX_CHANGED_PATHS = 512;
 
+	private static final PathDiffCalculator PATH_DIFF_CALCULATOR
+			= new PathDiffCalculator();
+
 	private final int hashsz;
 
 	private final GraphCommits graphCommits;
@@ -374,37 +377,6 @@ private void writeCommitData(CancellableDigestOutputStream out)
 		return generations;
 	}
 
-	private static Optional<HashSet<ByteBuffer>> computeBloomFilterPaths(
-			ObjectReader or, RevCommit cmit) throws MissingObjectException,
-			IncorrectObjectTypeException, CorruptObjectException, IOException {
-		HashSet<ByteBuffer> paths = new HashSet<>();
-		try (TreeWalk walk = new TreeWalk(null, or)) {
-			walk.setRecursive(true);
-			if (cmit.getParentCount() == 0) {
-				walk.addTree(new EmptyTreeIterator());
-			} else {
-				walk.addTree(cmit.getParent(0).getTree());
-			}
-			walk.addTree(cmit.getTree());
-			while (walk.next()) {
-				if (walk.idEqual(0, 1)) {
-					continue;
-				}
-				byte[] rawPath = walk.getRawPath();
-				paths.add(ByteBuffer.wrap(rawPath));
-				for (int i = 0; i < rawPath.length; i++) {
-					if (rawPath[i] == '/') {
-						paths.add(ByteBuffer.wrap(rawPath, 0, i));
-					}
-					if (paths.size() > MAX_CHANGED_PATHS) {
-						return Optional.empty();
-					}
-				}
-			}
-		}
-		return Optional.of(paths);
-	}
-
 	private BloomFilterChunks computeBloomFilterChunks(ProgressMonitor monitor)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			CorruptObjectException, IOException {
@@ -435,8 +407,8 @@ private BloomFilterChunks computeBloomFilterChunks(ProgressMonitor monitor)
 					filtersReused++;
 				} else {
 					filtersComputed++;
-					Optional<HashSet<ByteBuffer>> paths = computeBloomFilterPaths(
-							graphCommits.getObjectReader(), cmit);
+					Optional<HashSet<ByteBuffer>> paths = PATH_DIFF_CALCULATOR
+							.changedPaths(graphCommits.getObjectReader(), cmit);
 					if (paths.isEmpty()) {
 						cpf = ChangedPathFilter.FULL;
 					} else {
@@ -473,6 +445,40 @@ private void writeExtraEdges(CancellableDigestOutputStream out)
 		}
 	}
 
+	// Visible for testing
+	static class PathDiffCalculator {
+		Optional<HashSet<ByteBuffer>> changedPaths(
+				ObjectReader or, RevCommit cmit) throws MissingObjectException,
+				IncorrectObjectTypeException, CorruptObjectException, IOException {
+			HashSet<ByteBuffer> paths = new HashSet<>();
+			try (TreeWalk walk = new TreeWalk(null, or)) {
+				walk.setRecursive(true);
+				if (cmit.getParentCount() == 0) {
+					walk.addTree(new EmptyTreeIterator());
+				} else {
+					walk.addTree(cmit.getParent(0).getTree());
+				}
+				walk.addTree(cmit.getTree());
+				while (walk.next()) {
+					if (walk.idEqual(0, 1)) {
+						continue;
+					}
+					byte[] rawPath = walk.getRawPath();
+					paths.add(ByteBuffer.wrap(rawPath));
+					for (int i = 0; i < rawPath.length; i++) {
+						if (rawPath[i] == '/') {
+							paths.add(ByteBuffer.wrap(rawPath, 0, i));
+						}
+						if (paths.size() > MAX_CHANGED_PATHS) {
+							return Optional.empty();
+						}
+					}
+				}
+			}
+			return Optional.of(paths);
+		}
+	}
+
 	private static class ChunkHeader {
 		final int id;