Add option --orphan for checkout

Change-Id: I546a93f3e147d8d6fc70094b22679c0d11cd8921
Signed-off-by: Rüdiger Herrmann <ruediger.herrmann@gmx.de>

org.eclipse.jgit.pgm.test/tst/org/eclipse/jgit/pgm/CheckoutTest.java

@@ -188,6 +188,18 @@ public class CheckoutTest extends CLIRepositoryTestCase {
 		assertEquals("Hello world a", read(fileA));
 	}
 
+	@Test
+	public void testCheckoutOrphan() throws Exception {
+		Git git = new Git(db);
+		git.commit().setMessage("initial commit").call();
+
+		assertEquals("Switched to a new branch 'new_branch'",
+				execute("git checkout --orphan new_branch"));
+		assertEquals("refs/heads/new_branch", db.getRef("HEAD").getTarget().getName());
+		RevCommit commit = git.commit().setMessage("orphan commit").call();
+		assertEquals(0, commit.getParentCount());
+	}
+
 	/**
 	 * Steps:
 	 * <ol>

org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/internal/CLIText.properties

@@ -345,3 +345,4 @@ usage_updateRemoteRefsFromAnotherRepository=Update remote refs from another repo
 usage_useNameInsteadOfOriginToTrackUpstream=use <name> instead of 'origin' to track upstream
 usage_checkoutBranchAfterClone=checkout named branch instead of remotes's HEAD
 usage_viewCommitHistory=View commit history
+usage_orphan=Create a new orphan branch. The first commit made on this new branch will have no parents amd it will be the root of a new history totally disconnected from other branches and commits.

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Checkout.java

@@ -71,6 +71,9 @@ class Checkout extends TextBuiltin {
 	@Option(name = "--force", aliases = { "-f" }, usage = "usage_forceCheckout")
 	private boolean force = false;
 
+	@Option(name = "--orphan", usage = "usage_orphan")
+	private boolean orphan = false;
+
 	@Argument(required = true, index = 0, metaVar = "metaVar_name", usage = "usage_checkout")
 	private String name;
 
@@ -95,6 +98,7 @@ class Checkout extends TextBuiltin {
 			command.setCreateBranch(createBranch);
 			command.setName(name);
 			command.setForce(force);
+			command.setOrphan(orphan);
 		}
 		try {
 			String oldBranch = db.getBranch();
@@ -107,10 +111,9 @@ class Checkout extends TextBuiltin {
 						name));
 				return;
 			}
-			if (createBranch)
+			if (createBranch || orphan)
 				outw.println(MessageFormat.format(
-						CLIText.get().switchedToNewBranch,
-						Repository.shortenRefName(ref.getName())));
+						CLIText.get().switchedToNewBranch, name));
 			else
 				outw.println(MessageFormat.format(
 						CLIText.get().switchedToBranch,