Allow callers to request a specific type of manifest

If the caller knows exactly what the manifest type must be we
can now ask the loader to directly construct that type, rather
than guessing it from the working directory.

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

command.py

@@ -62,8 +62,10 @@ class Command(object):
   def manifest(self):
     return self.GetManifest()
 
-  def GetManifest(self, reparse=False):
-    return manifest_loader.GetManifest(self.repodir, reparse)
+  def GetManifest(self, reparse=False, type=None):
+    return manifest_loader.GetManifest(self.repodir,
+                                       reparse=reparse,
+                                       type=type)
 
   def GetProjects(self, args, missing_ok=False):
     """A list of projects that match the arguments.

manifest_loader.py

@@ -15,13 +15,17 @@
 
 from manifest_xml import XmlManifest
 
-def ParseManifest(repodir):
+def ParseManifest(repodir, type=None):
+  if type:
+    return type(repodir)
   return XmlManifest(repodir)
 
 _manifest = None
 
-def GetManifest(repodir, reparse=False):
+def GetManifest(repodir, reparse=False, type=None):
   global _manifest
-  if _manifest is None or reparse:
-    _manifest = ParseManifest(repodir)
+  if _manifest is None \
+  or reparse \
+  or (type and _manifest.__class__ != type):
+    _manifest = ParseManifest(repodir, type=type)
   return _manifest