Enable CommitCommand to use a fluent style

By returning `this` at the end of the `setCredentialsProvider()` the API
can be used as a fluent style.

This change is source compatible but not binary compatible with existing
clients, and so adding this will require a major version change.

Bug: 553116
Change-Id: I72eaefee72825fa2246319a94a0df3c0cb7061fc
Signed-off-by: Alex Blewitt <alex.blewitt@gmail.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

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

@@ -1019,10 +1019,12 @@ public class CommitCommand extends GitCommand<RevCommit> {
 	 * @param credentialsProvider
 	 *            the provider to use when querying for credentials (eg., during
 	 *            signing)
-	 * @since 5.3
+	 * @return {@code this}
+	 * @since 6.0
 	 */
-	public void setCredentialsProvider(
+	public CommitCommand setCredentialsProvider(
 			CredentialsProvider credentialsProvider) {
 		this.credentialsProvider = credentialsProvider;
+		return this;
 	}
 }