help: Fix help sync

help sync crashed as sync required the manifest to be configured to
create the option parser, as the default number of jobs is required.

Change-Id: Ie75e8d75ac0e38313e4aab451cbb24430e84def5
Signed-off-by: Shawn O. Pearce <sop@google.com>

subcmds/help.py

@@ -165,6 +165,7 @@ See 'repo help --all' for a complete list of recognized commands.
         print >>sys.stderr, "repo: '%s' is not a repo command." % name
         sys.exit(1)
 
+      cmd.manifest = self.manifest
       self._PrintCommandHelp(cmd)
 
     else: