Create an abstract Manifest base class

This will help as we add support for another manifest type.

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/subcmds/sync.py b/subcmds/sync.py
index bd07dd9..afd44da 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -214,7 +214,7 @@
         if not syncbuf.Finish():
           sys.exit(1)
 
-        self.manifest._Unload()
+        self.GetManifest(reparse=True)
         all = self.GetProjects(args, missing_ok=True)
         missing = []
         for project in all: