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>

main.py

@@ -32,11 +32,9 @@ from git_config import close_ssh
 from command import InteractiveCommand
 from command import MirrorSafeCommand
 from command import PagedCommand
-from editor import Editor
 from error import ManifestInvalidRevisionError
 from error import NoSuchProjectError
 from error import RepoChangedException
-from manifest_xml import XmlManifest
 from pager import RunPager
 
 from subcmds import all as all_commands
@@ -97,8 +95,6 @@ class _Repo(object):
       sys.exit(1)
 
     cmd.repodir = self.repodir
-    cmd.manifest = XmlManifest(cmd.repodir)
-    Editor.globalConfig = cmd.manifest.globalConfig
 
     if not isinstance(cmd, MirrorSafeCommand) and cmd.manifest.IsMirror:
       print >>sys.stderr, \