DirCacheCheckout: unlock DirCache in case of failure

DirCacheCheckout did not unlock the index if e.g. an IOException occured
during checkout.

Bug: 350677
Change-Id: Ie9fa09f7a404080da7cdccafb9be3a8c845e4869
Signed-off-by: Jens Baumgart <jens.baumgart@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCacheCheckout.java

@@ -375,6 +375,16 @@ public class DirCacheCheckout {
 	 * @throws IOException
 	 */
 	public boolean checkout() throws IOException {
+		try {
+			return doCheckout();
+		} finally {
+			dc.unlock();
+		}
+	}
+
+	private boolean doCheckout() throws CorruptObjectException, IOException,
+			MissingObjectException, IncorrectObjectTypeException,
+			CheckoutConflictException, IndexWriteException {
 		toBeDeleted.clear();
 		if (headCommitTree != null)
 			preScanTwoTrees();