Expose RefWriter constructor taking RefList

An implementation might prefer to use the RefList type here, and
RefList is part of our public API.  Expose the constructor so callers
who have a RefList can take advantage of the existing sorting.

Change-Id: I545867f85aa2c479d2d610024ebbe318144709c8
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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

@@ -86,8 +86,13 @@ public abstract class RefWriter {
 			this.refs = RefComparator.sort(refs.values());
 	}
 
-	RefWriter(RefList<Ref> list) {
-		this.refs = list.asList();
+	/**
+	 * @param refs
+	 *            the complete set of references. This should have been computed
+	 *            by applying updates to the advertised refs already discovered.
+	 */
+	public RefWriter(RefList<Ref> refs) {
+		this.refs = refs.asList();
 	}
 
 	/**