FetchCommand: do not set a null credentials provider

FetchCommand now does not set a null credentials provider on
Transport because in this case the default provider is replaced with
null and the default mechanism for providing credentials is not
working.

Change-Id: I44096aa856f031545df39d4b09af198caa2c21f6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/api/FetchCommand.java

@@ -128,6 +128,7 @@ public class FetchCommand extends GitCommand<FetchResult> {
 			if (tagOption != null)
 				transport.setTagOpt(tagOption);
 			transport.setFetchThin(thin);
+			if (credentialsProvider != null)
 				transport.setCredentialsProvider(credentialsProvider);
 
 			try {