Fix NPE when calling CreateBranch without explict startpoint

When creating a branch with CreateBranchCommand.call() without
specifying an explicit startPoint HEAD should be used as startPoint.
There was a bug leading to an NPE in such a case.

Change-Id: Ic0a5dc1f33a0987d66c09996c8012c45785500ff
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/BranchCommandTest.java

@@ -48,6 +48,7 @@ import org.eclipse.jgit.api.CreateBranchCommand.SetupUpstreamMode;
 import org.eclipse.jgit.api.ListBranchCommand.ListMode;
 import org.eclipse.jgit.api.errors.CannotDeleteCurrentBranchException;
 import org.eclipse.jgit.api.errors.DetachedHeadException;
+import org.eclipse.jgit.api.errors.GitAPIException;
 import org.eclipse.jgit.api.errors.InvalidRefNameException;
 import org.eclipse.jgit.api.errors.JGitInternalException;
 import org.eclipse.jgit.api.errors.NotMergedException;
@@ -400,6 +401,11 @@ public class BranchCommandTest extends RepositoryTestCase {
 		assertEquals(Constants.R_REMOTES + "newRemote", renamed.getName());
 	}
 
+	public void testCreationImplicitStart() throws JGitInternalException,
+			GitAPIException {
+		git.branchCreate().setName("topic").call();
+	}
+
 	public Ref createBranch(Git actGit, String name, boolean force,
 			String startPoint, SetupUpstreamMode mode)
 			throws JGitInternalException, RefAlreadyExistsException,

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

@@ -77,7 +77,7 @@ public class CreateBranchCommand extends GitCommand<Ref> {
 
 	private SetupUpstreamMode upstreamMode;
 
-	private String startPoint;
+	private String startPoint = Constants.HEAD;
 
 	private RevCommit startCommit;
 
@@ -275,9 +275,8 @@ public class CreateBranchCommand extends GitCommand<Ref> {
 			return startCommit.getId();
 		ObjectId result = null;
 		try {
-			if (startPoint == null)
-				result = repo.resolve(Constants.HEAD);
-			result = repo.resolve(startPoint);
+			result = repo.resolve((startPoint == null) ? Constants.HEAD
+					: startPoint);
 		} catch (AmbiguousObjectException e) {
 			throw e;
 		}