Restore default commit message

In 0.4.0, the plugin code was converted from Groovy to Java. As part of that, the wrong commit message string was copied. This resulted in the default changing in 0.4.0 (unintentionally) and a mismatch with the existing documentation.

This fixes #47.

src/main/java/org/ajoberstar/gradle/git/publish/GitPublishPlugin.java

@@ -24,7 +24,7 @@ public class GitPublishPlugin implements Plugin<Project> {
   public void apply(Project project) {
     GitPublishExtension extension = project.getExtensions().create("gitPublish", GitPublishExtension.class, project);
 
-    extension.getCommitMessage().set("Publish of Github pages from Gradle.");
+    extension.getCommitMessage().set("Generated by gradle-git-publish.");
 
     // if using the grgit plugin, default to the repo's origin
     project.getPluginManager().withPlugin("org.ajoberstar.grgit", plugin -> {