Add an optional flag to force an online reindex

As discussed on Gerrit ML[1] and in referenced issue, there are
situations where you may want to run an online reindex besides
the upgrade gerrit use case.  This change allows users to force an
online reindex.

[1] https://groups.google.com/d/msg/repo-discuss/pUn8fHVh58w/icsSFef2CAAJ

feature: issue 3987
Change-Id: Ie88b7effda08996f7c0f325543be703bcf09c794

gerrit-sshd/src/main/java/com/google/gerrit/sshd/commands/IndexStartCommand.java

@@ -23,11 +23,15 @@ import com.google.gerrit.sshd.SshCommand;
 import com.google.inject.Inject;
 
 import org.kohsuke.args4j.Argument;
+import org.kohsuke.args4j.Option;
 
 @RequiresCapability(GlobalCapability.ADMINISTRATE_SERVER)
 @CommandMetaData(name = "start", description = "Start the online reindexer")
 public class IndexStartCommand extends SshCommand {
 
+  @Option(name = "--force", usage = "force a re-index")
+  private boolean force;
+
   @Argument(index = 0, required = true, metaVar = "INDEX",
       usage = "index name to start")
   private String name;
@@ -38,7 +42,7 @@ public class IndexStartCommand extends SshCommand {
   @Override
   protected void run() throws UnloggedFailure {
     try {
-      if (luceneVersionManager.startReindexer(name)) {
+      if (luceneVersionManager.startReindexer(name, force)) {
         stdout.println("Reindexer started");
       } else {
         stdout.println("Nothing to reindex, index is already the latest version");