Expose the set of root commits in PackStatistics

Root commits are commits with zero parents.  If a commmit has no
parents it is the first commit in the repository.  In general the root
commits should be unique for any given project, as the first commit
will be created at a different time, by a different user with its own
message.  These root commits can be used as a "fingerprint" to
identify disjoint histories.

Change-Id: Id891dbc1f17c816cea404569578bb7635ff85cdb

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

@@ -1718,6 +1718,7 @@ public class PackWriter implements AutoCloseable {
 		final int maxBases = config.getDeltaSearchWindowSize();
 		Set<RevTree> baseTrees = new HashSet<RevTree>();
 		BlockList<RevCommit> commits = new BlockList<RevCommit>();
+		Set<ObjectId> roots = new HashSet<>();
 		RevCommit c;
 		while ((c = walker.next()) != null) {
 			if (exclude(c))
@@ -1729,8 +1730,12 @@ public class PackWriter implements AutoCloseable {
 			}
 
 			commits.add(c);
+			if (c.getParentCount() == 0) {
+				roots.add(c.copy());
+			}
 			countingMonitor.update(1);
 		}
+		stats.rootCommits = Collections.unmodifiableSet(roots);
 
 		if (shallowPack) {
 			for (RevCommit cmit : commits) {

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/PackStatistics.java

@@ -178,6 +178,9 @@ public class PackStatistics {
 		/** The collection of reused packs in the upload. */
 		public List<CachedPack> reusedPacks;
 
+		/** Commits with no parents. */
+		public Set<ObjectId> rootCommits;
+
 		/** If a shallow pack, the depth in commits. */
 		public int depth;
 
@@ -299,6 +302,11 @@ public class PackStatistics {
 		return statistics.reusedPacks;
 	}
 
+	/** @return unmodifiable collection of the root commits of the history. */
+	public Set<ObjectId> getRootCommits() {
+		return statistics.rootCommits;
+	}
+
 	/**
 	 * @return number of objects in the output pack that went through the delta
 	 *         search process in order to find a potential delta base.