Handle null in ProgressMonitor setters

These commands' monitor fields can never be null unless someone passes
null to setProgressMonitor.  Anyone passing null probably meant to
disable the ProgressMonitor, so do that (by falling back to
NullProgressMonitor.INSTANCE) instead of saving a null and eventually
producing NullPointerException.

Change-Id: I63ad93ea8ad669fd333a5fd40880e7583ba24827
Signed-off-by: Jonathan Nieder <jrn@google.com>

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

 	 * @return {@code this}
 	 */
 	public CloneCommand setProgressMonitor(ProgressMonitor monitor) {
+		if (monitor == null) {
+			monitor = NullProgressMonitor.INSTANCE;
+		}
 		this.monitor = monitor;
 		return this;
 	}

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

 	 * @return this instance
 	 */
 	public DiffCommand setProgressMonitor(ProgressMonitor monitor) {
+		if (monitor == null) {
+			monitor = NullProgressMonitor.INSTANCE;
+		}
 		this.monitor = monitor;
 		return this;
 	}
-}
+}

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

 	 */
 	public FetchCommand setProgressMonitor(ProgressMonitor monitor) {
 		checkCallable();
+		if (monitor == null) {
+			monitor = NullProgressMonitor.INSTANCE;
+		}
 		this.monitor = monitor;
 		return this;
 	}

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

 	 * @return this instance
 	 */
 	public PullCommand setProgressMonitor(ProgressMonitor monitor) {
+		if (monitor == null) {
+			monitor = NullProgressMonitor.INSTANCE;
+		}
 		this.monitor = monitor;
 		return this;
 	}

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

 	 */
 	public PushCommand setProgressMonitor(ProgressMonitor monitor) {
 		checkCallable();
+		if (monitor == null) {
+			monitor = NullProgressMonitor.INSTANCE;
+		}
 		this.monitor = monitor;
 		return this;
 	}

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

 	 * @return this instance
 	 */
 	public RebaseCommand setProgressMonitor(ProgressMonitor monitor) {
+		if (monitor == null) {
+			monitor = NullProgressMonitor.INSTANCE;
+		}
 		this.monitor = monitor;
 		return this;
 	}