Update ORIG_HEAD when resetting

Write the old object id from the RefUpdate to the
ORIG_HEAD file after the update completes.

Add two new convenience methods to Repository to read
and write the ORIG_HEAD reference similar to the methods
for reading/writing CHERRY_PICK_HEAD and MERGE_HEAD.

Bug: 375525
Change-Id: I120b3b2cd3b1ddae88fce435285bae15cbf96f5e

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

 		// change third line in topic branch
 		writeTrashFile(FILE1, "1\n2\n3\ntopic\n");
 		git.add().addFilepattern(FILE1).call();
-		git.commit().setMessage("change file1 in topic").call();
+		RevCommit origHead = git.commit().setMessage("change file1 in topic")
+				.call();
 
 		RebaseResult res = git.rebase().setUpstream("refs/heads/master").call();
 		assertEquals(Status.OK, res.getStatus());
 		assertEquals("refs/heads/topic", db.getFullBranch());
 		assertEquals(lastMasterChange, new RevWalk(db).parseCommit(
 				db.resolve(Constants.HEAD)).getParent(0));
+		assertEquals(origHead, db.readOrigHead());
 	}
 
 	@Test

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

 		assertFalse(inHead(fileInIndexPath));
 		assertFalse(inIndex(indexFile.getName()));
 		assertReflog(prevHead, head);
+		assertEquals(prevHead, db.readOrigHead());
 	}
 
 	@Test
 		assertFalse(inHead(fileInIndexPath));
 		assertTrue(inIndex(indexFile.getName()));
 		assertReflog(prevHead, head);
+		assertEquals(prevHead, db.readOrigHead());
 	}
 
 	@Test
 		assertFalse(inIndex(indexFile.getName()));
 
 		assertReflog(prevHead, head);
+		assertEquals(prevHead, db.readOrigHead());
 	}
 
 	@Test

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

 		// create the folder for the meta information
 		FileUtils.mkdir(rebaseDir);
 
-		createFile(repo.getDirectory(), Constants.ORIG_HEAD, headId.name());
+		repo.writeOrigHead(headId);
 		createFile(rebaseDir, REBASE_HEAD, headId.name());
 		createFile(rebaseDir, HEAD_NAME, headName);
 		createFile(rebaseDir, ONTO, upstreamCommit.name());
 
 	private RebaseResult abort(RebaseResult result) throws IOException {
 		try {
-			String commitId = readFile(repo.getDirectory(), Constants.ORIG_HEAD);
+			ObjectId origHead = repo.readOrigHead();
+			String commitId = origHead != null ? origHead.name() : null;
 			monitor.beginTask(MessageFormat.format(
 					JGitText.get().abortingRebase, commitId),
 					ProgressMonitor.UNKNOWN);

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

 				throw new JGitInternalException(MessageFormat.format(
 						JGitText.get().cannotLock, ru.getName()));
 
+			ObjectId origHead = ru.getOldObjectId();
+			if (origHead != null)
+				repo.writeOrigHead(origHead);
+
 			switch (mode) {
 				case HARD:
 					checkoutIndex(commit);

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

 		writeHeadsFile(heads, Constants.CHERRY_PICK_HEAD);
 	}
 
+	/**
+	 * Write original HEAD commit into $GIT_DIR/ORIG_HEAD.
+	 *
+	 * @param head
+	 *            an object id of the original HEAD commit or <code>null</code>
+	 *            to delete the file
+	 * @throws IOException
+	 */
+	public void writeOrigHead(ObjectId head) throws IOException {
+		List<ObjectId> heads = head != null ? Collections.singletonList(head)
+				: null;
+		writeHeadsFile(heads, Constants.ORIG_HEAD);
+	}
+
+	/**
+	 * Return the information stored in the file $GIT_DIR/ORIG_HEAD.
+	 *
+	 * @return object id from ORIG_HEAD file or {@code null} if this file
+	 *         doesn't exist. Also if the file exists but is empty {@code null}
+	 *         will be returned
+	 * @throws IOException
+	 * @throws NoWorkTreeException
+	 *             if this is bare, which implies it has no working directory.
+	 *             See {@link #isBare()}.
+	 */
+	public ObjectId readOrigHead() throws IOException, NoWorkTreeException {
+		if (isBare() || getDirectory() == null)
+			throw new NoWorkTreeException();
+
+		byte[] raw = readGitDirectoryFile(Constants.ORIG_HEAD);
+		return raw != null ? ObjectId.fromString(raw, 0) : null;
+	}
+
 	/**
 	 * Read a file from the git directory.
 	 *