Fixed javadoc: the getBranch() and getFullBranch() can return null

Change-Id: I284a1b5f8220f68496f992b0e97e6934e03ae616
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -849,8 +849,9 @@ public abstract class Repository implements AutoCloseable {
 	 * Except when HEAD is detached, in which case this method returns the
 	 * current ObjectId in hexadecimal string format.
 	 *
-	 * @return name of current branch (for example {@code refs/heads/master}) or
+	 * @return name of current branch (for example {@code refs/heads/master}),
-	 *         an ObjectId in hex format if the current branch is detached.
+	 *         an ObjectId in hex format if the current branch is detached,
+	 *         or null if the repository is corrupt and has no HEAD reference.
 	 * @throws IOException
 	 */
 	public String getFullBranch() throws IOException {
@@ -871,8 +872,9 @@ public abstract class Repository implements AutoCloseable {
 	 * leading prefix {@code refs/heads/} is removed from the reference before
 	 * it is returned to the caller.
 	 *
-	 * @return name of current branch (for example {@code master}), or an
+	 * @return name of current branch (for example {@code master}), an
-	 *         ObjectId in hex format if the current branch is detached.
+	 *         ObjectId in hex format if the current branch is detached,
+	 *         or null if the repository is corrupt and has no HEAD reference.
 	 * @throws IOException
 	 */
 	public String getBranch() throws IOException {