Init: Accept -b as an alias of --batch

Change-Id: Id22c6c8a7fc407cb8da7e3f64299648714a5d78a

Documentation/pgm-init.txt

@@ -30,6 +30,7 @@ If run in an existing `$site_path`, init upgrades existing resources
 (e.g. DB schema, plugins) as necessary.
 
 == OPTIONS
+-b::
 --batch::
 	Run in batch mode, skipping interactive prompts. For a fresh
 	install, reasonable configuration defaults are chosen based

gerrit-pgm/src/main/java/com/google/gerrit/pgm/Init.java

@@ -45,7 +45,8 @@ import java.util.List;
 
 /** Initialize a new Gerrit installation. */
 public class Init extends BaseInit {
-  @Option(name = "--batch", usage = "Batch mode; skip interactive prompting")
+  @Option(name = "--batch", aliases = {"-b"},
+      usage = "Batch mode; skip interactive prompting")
   private boolean batchMode;
 
   @Option(name = "--no-auto-start", usage = "Don't automatically start daemon after init")