Fix persistent-https relative url resolving

Previously, we would remove 'persistent-' then tack it on at the end
if it had been previously found.  However, this would ignore urljoin's
decision on whether or not the second path was relative.  Instead, we
were always assuming it was relative and that we didn't want to use
a different absolute url with a different protocol.

This change handles persistent-https:// in the same way we handled the
absense of an explicit protocol.  The only difference is that this time
instead of temporarily replacing it with 'gopher://', we use 'wais://'.

Change-Id: I6e8ad1eb4b911931a991481717f1ade01315db2a

manifest_xml.py

@@ -80,18 +80,20 @@ class _XmlRemote(object):
   def _resolveFetchUrl(self):
     url = self.fetchUrl.rstrip('/')
     manifestUrl = self.manifestUrl.rstrip('/')
-    p = manifestUrl.startswith('persistent-http')
-    if p:
-      manifestUrl = manifestUrl[len('persistent-'):]
-
-    # urljoin will get confused if there is no scheme in the base url
-    # ie, if manifestUrl is of the form <hostname:port>
+    # urljoin will gets confused over quite a few things.  The ones we care
+    # about here are:
+    # * no scheme in the base url, like <hostname:port>
+    # * persistent-https://
+    # We handle this by replacing these with obscure protocols
+    # and then replacing them with the original when we are done.
+    # gopher -> <none>
+    # wais -> persistent-https
     if manifestUrl.find(':') != manifestUrl.find('/') - 1:
       manifestUrl = 'gopher://' + manifestUrl
+    manifestUrl = re.sub(r'^persistent-https://', 'wais://', manifestUrl)
     url = urllib.parse.urljoin(manifestUrl, url)
     url = re.sub(r'^gopher://', '', url)
-    if p:
-      url = 'persistent-' + url
+    url = re.sub(r'^wais://', 'persistent-https://', url)
     return url
 
   def ToRemoteSpec(self, projectName):