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/init.py

@@ -86,6 +86,10 @@ to update the working directory files.
     g.add_option('--depth', type='int', default=None,
                  dest='depth',
                  help='create a shallow clone with given depth; see git clone')
+    g.add_option('-g', '--groups',
+                 dest='groups', default="",
+                 help='restrict manifest projects to ones with a specified group',
+                 metavar='GROUP')
 
     # Tool
     g = p.add_option_group('repo Version options')
@@ -135,6 +139,8 @@ to update the working directory files.
       r.ResetFetch()
       r.Save()
 
+    m.config.SetString('manifest.groups', opt.groups)
+
     if opt.reference:
       m.config.SetString('repo.reference', opt.reference)