Fix SshSessionFactory#setInstance to use service loader

If setInstance(SshSessionFactory) is called with parameter null
set default session factory using the newly introduced service loader
instead of hard-coding the default factory class.

Change-Id: I86b5932333cc53b706534a2822e0fd96e12e6e47
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/transport/SshSessionFactory.java

@@ -63,10 +63,11 @@ public abstract class SshSessionFactory {
 	 *            default factory will be restored.s
 	 */
 	public static void setInstance(SshSessionFactory newFactory) {
-		if (newFactory != null)
+		if (newFactory != null) {
 			INSTANCE = newFactory;
-		else
-			INSTANCE = new DefaultSshSessionFactory();
+		} else {
+			INSTANCE = loadSshSessionFactory();
+		}
 	}
 
 	/**