Add manifest groups

Allows specifying a list of groups with a -g argument to repo init.
The groups act on a group= attribute specified on projects in the
manifest.
All projects are implicitly labelled with "default" unless they are
explicitly labelled "-default".
Prefixing a group with "-" removes matching projects from the list
of projects to sync.
If any non-inverted manifest groups are specified, the default label
is ignored.

Change-Id: I3a0dd7a93a8a1756205de1d03eee8c00906af0e5
Reviewed-on: https://gerrit-review.googlesource.com/34570
Reviewed-by: Shawn Pearce <sop@google.com>
Tested-by: Shawn Pearce <sop@google.com>

subcmds/sync.py

@@ -277,7 +277,7 @@ later is required to fix a server side protocol bug.
 
   def UpdateProjectList(self):
     new_project_paths = []
-    for project in self.manifest.projects.values():
+    for project in self.GetProjects(None, missing_ok=True):
       if project.relpath:
         new_project_paths.append(project.relpath)
     file_name = 'project.list'
@@ -306,7 +306,8 @@ later is required to fix a server side protocol bug.
                              worktree = os.path.join(self.manifest.topdir, path),
                              relpath = path,
                              revisionExpr = 'HEAD',
-                             revisionId = None)
+                             revisionId = None,
+                             groups = None)
 
               if project.IsDirty():
                 print >>sys.stderr, 'error: Cannot remove project "%s": \