Throw JGit exception when ResetCommand got wrong ref

If the ResetCommand should reset to a invalid ref (e.g. HEAD in a repo
whithout a single commit) it was throwing an NPE. This is fixed now by
throwing a JGitInternalExcpeption. It would be nicer if we could throw
a InvalidRefException, but this would modify our API.

Bug: 339610
Change-Id: Iffcb4f2cca9f702176471d93c3a71e5cb3e700b1
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

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

@@ -46,6 +46,7 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
 
 import java.io.File;
 import java.io.IOException;
@@ -156,6 +157,22 @@ public class ResetCommandTest extends RepositoryTestCase {
 		assertReflog(prevHead, head);
 	}
 
+	@Test
+	public void testResetToNonexistingHEAD() throws JGitInternalException,
+			AmbiguousObjectException, IOException {
+
+		// create a file in the working tree of a fresh repo
+		git = new Git(db);
+		writeTrashFile("f", "content");
+
+		try {
+			git.reset().setRef(Constants.HEAD).call();
+			fail("Expected JGitInternalException didn't occur");
+		} catch (JGitInternalException e) {
+			// got the expected exception
+		}
+	}
+
 	@Test
 	public void testSoftReset() throws JGitInternalException,
 			AmbiguousObjectException, IOException, NoFilepatternException,

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

@@ -151,6 +151,12 @@ public class ResetCommand extends GitCommand<Ref> {
 			final ObjectId commitId;
 			try {
 				commitId = repo.resolve(ref);
+				if (commitId == null) {
+					// @TODO throw an InvalidRefNameException. We can't do that
+					// now because this would break the API
+					throw new JGitInternalException("Invalid ref " + ref
+							+ " specified");
+				}
 			} catch (IOException e) {
 				throw new JGitInternalException(
 						MessageFormat.format(JGitText.get().cannotRead, ref),