Allow applications to pass ObjectInserter to Merger

Gerrit Code Review needs to control which inserter is used by a
Merger. Allow the application to set the inserter before calling
merge, giving callers more direct control over how objects will
be created.

Change-Id: I3c527a493db4659e95289ff3077cffb9e32336cf

org.eclipse.jgit/src/org/eclipse/jgit/merge/Merger.java

@@ -108,6 +108,8 @@ public abstract class Merger {
 
 	/**
 	 * @return an object writer to create objects in {@link #getRepository()}.
+	 *         If no inserter has been set on this instance, one will be created
+	 *         and returned by all future calls.
 	 */
 	public ObjectInserter getObjectInserter() {
 		if (inserter == null)
@@ -115,6 +117,23 @@ public abstract class Merger {
 		return inserter;
 	}
 
+	/**
+	 * Set the inserter this merger will use to create objects.
+	 * <p>
+	 * If an inserter was already set on this instance (such as by a prior set,
+	 * or a prior call to {@link #getObjectInserter()}), the prior inserter will
+	 * be released first.
+	 *
+	 * @param oi
+	 *            the inserter instance to use. Must be associated with the
+	 *            repository instance returned by {@link #getRepository()}.
+	 */
+	public void setObjectInserter(ObjectInserter oi) {
+		if (inserter != null)
+			inserter.release();
+		inserter = oi;
+	}
+
 	/**
 	 * Merge together two or more tree-ish objects.
 	 * <p>