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>
diff --git a/subcmds/help.py b/subcmds/help.py
index 90b817d..0df3c14 100644
--- a/subcmds/help.py
+++ b/subcmds/help.py
@@ -165,6 +165,7 @@
         print >>sys.stderr, "repo: '%s' is not a repo command." % name
         sys.exit(1)
 
+      cmd.manifest = self.manifest
       self._PrintCommandHelp(cmd)
 
     else: