Sync: Add HTTP Cookie File header on temporary cookie file

The .gitcookies file generated by googlesource.com does not have
the header:

 # (Netscape) HTTP Cookie File

which causes python's MozillaCookieJar.load to fail with the
error:

 "does not look like a Netscape format cookies file"

Prepend the expected header onto the generated cookie file.

We don't bother to check if the header already exists on the
file; repeating it does not cause any problem.

Bug: Issue 207
Change-Id: I7d39720a1d36a6aae00f70691156514ebc04e579

subcmds/sync.py

@@ -916,6 +916,7 @@ class PersistentTransport(xmlrpc.client.Transport):
       # stripping those prefixes away.
       if cookiefile:
         tmpcookiefile = tempfile.NamedTemporaryFile()
+        tmpcookiefile.write("# HTTP Cookie File")
         try:
           with open(cookiefile) as f:
             for line in f: