Fix ResetCommand to default to mixed reset

ResetCommand threw an NPE if neither mode nor path was defined. Instead
it should default to a mixed reset like native git does.

Change-Id: I455902394f9e7b0c7afae42381f34838f7f2a138
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -424,6 +424,21 @@ public class ResetCommandTest extends RepositoryTestCase {
 		git.reset().setRef("doesnotexist").addPath("a.txt").call();
 	}
 
+	@Test
+	public void testResetDefaultMode() throws Exception {
+		git = new Git(db);
+		writeTrashFile("a.txt", "content");
+		git.add().addFilepattern("a.txt").call();
+		writeTrashFile("a.txt", "modified");
+		// should use default mode MIXED
+		git.reset().call();
+
+		DirCache cache = db.readDirCache();
+		DirCacheEntry aEntry = cache.getEntry("a.txt");
+		assertNull(aEntry);
+		assertEquals("modified", read("a.txt"));
+	}
+
 	@Test
 	public void testHardResetOnTag() throws Exception {
 		setupRepository();

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

@@ -195,6 +195,9 @@ public class ResetCommand extends GitCommand<Ref> {
 				result = repo.getRef(Constants.HEAD);
 			}
 
+			if (mode == null)
+				mode = ResetType.MIXED;
+
 			switch (mode) {
 				case HARD:
 					checkoutIndex(commitTree);