Fix committing empty commits

Allow to explicitly create an empty commit even if committing only
certain files.

Bug: 510685 
Change-Id: If9bf664d7cd824f8e5bd6765fa6cc739af3d7721

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

@@ -557,6 +557,11 @@ public class CommitCommandTest extends RepositoryTestCase {
 			} catch (EmtpyCommitException e) {
 				// expect this exception
 			}
+
+			// Allow empty commits also when setOnly was set
+			git.commit().setAuthor("New Author", "newauthor@example.org")
+					.setMessage("again no change").setOnly("file1")
+					.setAllowEmpty(true).call();
 		}
 	}
 

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

@@ -482,7 +482,7 @@ public class CommitCommand extends GitCommand<RevCommit> {
 						JGitText.get().entryNotFoundByPath, only.get(i)));
 
 		// there must be at least one change
-		if (emptyCommit)
+		if (emptyCommit && !allowEmpty.booleanValue())
 			// Would like to throw a EmptyCommitException. But this would break the API
 			// TODO(ch): Change this in the next release
 			throw new JGitInternalException(JGitText.get().emptyCommit);