Make pull --rebase on an unborn branch do a checkout

A merging pull on an unborn branch was already supported. But a
rebasing pull failed. If the user has pull.rebase = true in his
user config, the pull would try to rebase. Rebasing needs a parent
commit, though. Native git handles this case:

  git init
  git remote add origin <URI>
  git pull --rebase origin master

Check up front in PullCommand for the unborn head and just do a
checkout in this case. MergeCommand already has similar code.

Bug: 544965
Change-Id: I1277e1ac0b0364b4623fd791f3d6b07bd5f58fca
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

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

@@ -718,6 +718,34 @@ public class CloneCommandTest extends RepositoryTestCase {
 
 	}
 
+	@Test
+	public void testCloneWithPullMerge() throws Exception {
+		File directory = createTempDirectory("testCloneRepository1");
+		try (Git g = Git.init().setDirectory(directory).setBare(false).call()) {
+			g.remoteAdd().setName(Constants.DEFAULT_REMOTE_NAME)
+					.setUri(new URIish(fileUri())).call();
+			PullResult result = g.pull().setRebase(false).call();
+			assertTrue(result.isSuccessful());
+			assertEquals("refs/heads/master",
+					g.getRepository().getFullBranch());
+			checkFile(new File(directory, "Test.txt"), "Hello world");
+		}
+	}
+
+	@Test
+	public void testCloneWithPullRebase() throws Exception {
+		File directory = createTempDirectory("testCloneRepository1");
+		try (Git g = Git.init().setDirectory(directory).setBare(false).call()) {
+			g.remoteAdd().setName(Constants.DEFAULT_REMOTE_NAME)
+					.setUri(new URIish(fileUri())).call();
+			PullResult result = g.pull().setRebase(true).call();
+			assertTrue(result.isSuccessful());
+			assertEquals("refs/heads/master",
+					g.getRepository().getFullBranch());
+			checkFile(new File(directory, "Test.txt"), "Hello world");
+		}
+	}
+
 	private String fileUri() {
 		return "file://" + git.getRepository().getWorkTree().getAbsolutePath();
 	}

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

@@ -60,6 +60,7 @@ import org.eclipse.jgit.api.errors.NoHeadException;
 import org.eclipse.jgit.api.errors.RefNotAdvertisedException;
 import org.eclipse.jgit.api.errors.RefNotFoundException;
 import org.eclipse.jgit.api.errors.WrongRepositoryStateException;
+import org.eclipse.jgit.dircache.DirCacheCheckout;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.AnyObjectId;
 import org.eclipse.jgit.lib.BranchConfig.BranchRebaseMode;
@@ -67,12 +68,17 @@ import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ConfigConstants;
 import org.eclipse.jgit.lib.Constants;
 import org.eclipse.jgit.lib.NullProgressMonitor;
+import org.eclipse.jgit.lib.ObjectId;
 import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefUpdate;
+import org.eclipse.jgit.lib.RefUpdate.Result;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.lib.RepositoryState;
 import org.eclipse.jgit.lib.SubmoduleConfig.FetchRecurseSubmodulesMode;
 import org.eclipse.jgit.merge.MergeStrategy;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.transport.FetchResult;
 import org.eclipse.jgit.transport.TagOpt;
 
@@ -339,6 +345,45 @@ public class PullCommand extends TransportCommand<PullCommand, PullResult> {
 
 		PullResult result;
 		if (pullRebaseMode != BranchRebaseMode.NONE) {
+			try {
+				Ref head = repo.exactRef(Constants.HEAD);
+				if (head == null) {
+					throw new NoHeadException(JGitText
+							.get().commitOnRepoWithoutHEADCurrentlyNotSupported);
+				}
+				ObjectId headId = head.getObjectId();
+				if (headId == null) {
+					// Pull on an unborn branch: checkout
+					try (RevWalk revWalk = new RevWalk(repo)) {
+						RevCommit srcCommit = revWalk
+								.parseCommit(commitToMerge);
+						DirCacheCheckout dco = new DirCacheCheckout(repo,
+								repo.lockDirCache(), srcCommit.getTree());
+						dco.setFailOnConflict(true);
+						dco.setProgressMonitor(monitor);
+						dco.checkout();
+						RefUpdate refUpdate = repo
+								.updateRef(head.getTarget().getName());
+						refUpdate.setNewObjectId(commitToMerge);
+						refUpdate.setExpectedOldObjectId(null);
+						refUpdate.setRefLogMessage("initial pull", false); //$NON-NLS-1$
+						if (refUpdate.update() != Result.NEW) {
+							throw new NoHeadException(JGitText
+									.get().commitOnRepoWithoutHEADCurrentlyNotSupported);
+						}
+						monitor.endTask();
+						return new PullResult(fetchRes, remote,
+								RebaseResult.result(
+										RebaseResult.Status.FAST_FORWARD,
+										srcCommit));
+					}
+				}
+			} catch (NoHeadException e) {
+				throw e;
+			} catch (IOException e) {
+				throw new JGitInternalException(JGitText
+						.get().exceptionCaughtDuringExecutionOfPullCommand, e);
+			}
 			RebaseCommand rebase = new RebaseCommand(repo);
 			RebaseResult rebaseRes = rebase.setUpstream(commitToMerge)
 					.setUpstreamName(upstreamName).setProgressMonitor(monitor)