Allow meta projects to be created not under .repo/

Some types of manifests might prefer to put their meta project work
tree under topdir, rather than inside of the .repo/ directory.  We
can support that by allowing relpath to be optionally passed in.

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

project.py

@@ -1426,15 +1426,17 @@ class SyncBuffer(object):
 class MetaProject(Project):
   """A special project housed under .repo.
   """
-  def __init__(self, manifest, name, gitdir, worktree):
+  def __init__(self, manifest, name, gitdir, worktree, relpath=None):
     repodir = manifest.repodir
+    if relpath is None:
+      relpath = '.repo/%s' % name
     Project.__init__(self,
                      manifest = manifest,
                      name = name,
                      gitdir = gitdir,
                      worktree = worktree,
                      remote = RemoteSpec('origin'),
-                     relpath = '.repo/%s' % name,
+                     relpath = relpath,
                      revisionExpr = 'refs/heads/master',
                      revisionId = None)