change help to -V

running 'p4 help' outside of Perforce repository makes it fail

Author: vladak

opengrok-indexer/src/main/java/org/opengrok/indexer/history/PerforceRepository.java

@@ -210,7 +210,7 @@ boolean isRepositoryFor(File file, CommandTimeoutType cmdType) {
 
     private static boolean isP4Working() {
         String repoCommand = getCommand(PerforceRepository.class, CMD_PROPERTY_KEY, CMD_FALLBACK);
-        boolean works = checkCmd(repoCommand, "help");
+        boolean works = checkCmd(repoCommand, "-V");
         LOGGER.log(Level.WARNING, "Command ''{0}'' does not work. " +
                 "Some operations with Perforce repositories will fail as a result.", repoCommand);
         return works;