RebaseCommand: fix stopping on root commit conflicts

If rebasing runs into a conflict when applying a root commit from an
independent branch, there is no parent commit. Write an empty patch
file in that case like C git does.

Bug: jgit-6
Change-Id: I315313673d2abf29639d7d96c958d599961ba276
Signed-off-by: Thomas Wolf <twolf@apache.org>

2 files changed, 43 insertions, 5 deletions

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RebaseCommandTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RebaseCommandTest.java
index 16f7cd1eb0..987c98e232 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RebaseCommandTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/RebaseCommandTest.java
@@ -332,6 +332,40 @@ public class RebaseCommandTest extends RepositoryTestCase {
 	}
 
 	/**
+	 * Create a commit A and an unrelated commit B creating the same file with
+	 * different content. Then rebase A onto B. The rebase should stop with a
+	 * conflict.
+	 *
+	 * @throws Exception
+	 *             on errors
+	 */
+	@Test
+	public void testRebaseNoMergeBaseConflict() throws Exception {
+		writeTrashFile(FILE1, FILE1);
+		git.add().addFilepattern(FILE1).call();
+		RevCommit first = git.commit().setMessage("Add file").call();
+		File file1 = new File(db.getWorkTree(), FILE1);
+		assertTrue(file1.exists());
+		// Create an independent branch
+		git.checkout().setOrphan(true).setName("orphan").call();
+		git.rm().addFilepattern(FILE1).call();
+		assertFalse(file1.exists());
+		writeTrashFile(FILE1, "something else");
+		git.add().addFilepattern(FILE1).call();
+		git.commit().setMessage("Orphan").call();
+		checkoutBranch("refs/heads/master");
+		assertEquals(first.getId(), db.resolve("HEAD"));
+		RebaseResult res = git.rebase().setUpstream("refs/heads/orphan").call();
+		assertEquals(Status.STOPPED, res.getStatus());
+		assertEquals(first, res.getCurrentCommit());
+		checkFile(file1, "<<<<<<< Upstream, based on orphan\n"
+				+ "something else\n"
+				+ "=======\n"
+				+ "file1\n"
+				+ ">>>>>>> " + first.abbreviate(7).name() + " Add file\n");
+	}
+
+	/**
 	 * Create the following commits and then attempt to rebase topic onto
 	 * master. This will serialize the branches.
 	 *
diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/RebaseCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/RebaseCommand.java
index 0f7b8afe21..757aff89c3 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/RebaseCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/RebaseCommand.java
@@ -1064,12 +1064,16 @@ public class RebaseCommand extends GitCommand<RebaseResult> {
 		String authorScript = toAuthorScript(author);
 		rebaseState.createFile(AUTHOR_SCRIPT, authorScript);
 		rebaseState.createFile(MESSAGE, commitToPick.getFullMessage());
-		ByteArrayOutputStream bos = new ByteArrayOutputStream();
-		try (DiffFormatter df = new DiffFormatter(bos)) {
-			df.setRepository(repo);
-			df.format(commitToPick.getParent(0), commitToPick);
+		if (commitToPick.getParentCount() > 0) {
+			ByteArrayOutputStream bos = new ByteArrayOutputStream();
+			try (DiffFormatter df = new DiffFormatter(bos)) {
+				df.setRepository(repo);
+				df.format(commitToPick.getParent(0), commitToPick);
+			}
+			rebaseState.createFile(PATCH, new String(bos.toByteArray(), UTF_8));
+		} else {
+			rebaseState.createFile(PATCH, ""); //$NON-NLS-1$
 		}
-		rebaseState.createFile(PATCH, new String(bos.toByteArray(), UTF_8));
 		rebaseState.createFile(STOPPED_SHA,
 				repo.newObjectReader()
 				.abbreviate(