Add --help to GerritLauncher

The launcher did not support any help switches, yet it did have a help
message which was output when no switches were supplied. Add a quick
--help switch and -h shorthand to make the launcher a bit friendlier
and safer to probe.

Change-Id: I6a6acfbaafc176894fbf4cbeb51452ea8018cf86
(cherry picked from commit 4b385d884d)

java/com/google/gerrit/launcher/GerritLauncher.java

@@ -78,7 +78,7 @@ public final class GerritLauncher {
    * @throws Exception if any error occurs.
    */
   public static int mainImpl(String[] argv) throws Exception {
-    if (argv.length == 0) {
+    if (argv.length == 0 || "-h".equals(argv[0]) || "--help".equals(argv[0])) {
       File me;
       try {
         me = getDistributionArchive();