Allow to amend a commit with CommitCommand

Bug: 339088
Change-Id: I57dc727688c4bb6968ac076b176661c857c05afa
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

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

@@ -238,4 +238,21 @@ public class CommitAndLogCommandTests extends RepositoryTestCase {
 		}
 		assertEquals(l, -1);
 	}
+
+	@Test
+	public void testCommitAmend() throws NoHeadException, NoMessageException,
+			UnmergedPathException, ConcurrentRefUpdateException,
+			JGitInternalException, WrongRepositoryStateException {
+		Git git = new Git(db);
+		git.commit().setMessage("first comit").call(); // typo
+		git.commit().setAmend(true).setMessage("first commit").call();
+
+		Iterable<RevCommit> commits = git.log().call();
+		int c = 0;
+		for (RevCommit commit : commits) {
+			assertEquals("first commit", commit.getFullMessage());
+			c++;
+		}
+		assertEquals(1, c);
+	}
 }

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

@@ -87,6 +87,8 @@ public class CommitCommand extends GitCommand<RevCommit> {
 
 	private boolean all;
 
+	private boolean amend;
+
 	/**
 	 * parents this commit should have. The current HEAD will be in this list
 	 * and also all commits mentioned in .git/MERGE_HEAD
@@ -155,7 +157,15 @@ public class CommitCommand extends GitCommand<RevCommit> {
 			// determine the current HEAD and the commit it is referring to
 			ObjectId headId = repo.resolve(Constants.HEAD + "^{commit}");
 			if (headId != null)
-				parents.add(0, headId);
+				if (amend) {
+					RevCommit previousCommit = new RevWalk(repo)
+							.parseCommit(headId);
+					RevCommit[] p = previousCommit.getParents();
+					for (int i = 0; i < p.length; i++)
+						parents.add(0, p[i].getId());
+				} else {
+					parents.add(0, headId);
+				}
 
 			// lock the index
 			DirCache index = repo.lockDirCache();
@@ -187,9 +197,10 @@ public class CommitCommand extends GitCommand<RevCommit> {
 								+ revCommit.getShortMessage(), false);
 
 						ru.setExpectedOldObjectId(headId);
-						Result rc = ru.update();
+						Result rc = ru.forceUpdate();
 						switch (rc) {
 						case NEW:
+						case FORCED:
 						case FAST_FORWARD: {
 							setCallable(false);
 							if (state == RepositoryState.MERGING_RESOLVED) {
@@ -387,4 +398,17 @@ public class CommitCommand extends GitCommand<RevCommit> {
 		return this;
 	}
 
+	/**
+	 * Used to amend the tip of the current branch. If set to true, the previous
+	 * commit will be amended. This is equivalent to --amend on the command
+	 * line.
+	 *
+	 * @param amend
+	 * @return {@code this}
+	 */
+	public CommitCommand setAmend(boolean amend) {
+		this.amend = amend;
+		return this;
+	}
+
 }