Consider local project to be default for 'repo start'

The requirement to explicitly specify the local project when starting
a new repo branch is somewhat counter intuitive.

This patch uses the current directory's git tree as the default
project.

Tested by running

  'repo start <name>'

observed that the result is the same as if running

  'repo start <name> .'

Change-Id: If106caa801b4cd5ba70dbe8354a227d59f100aa3

subcmds/start.py

@@ -54,8 +54,7 @@ revision specified in the manifest.
     if not opt.all:
       projects = args[1:]
       if len(projects) < 1:
-        print("error: at least one project must be specified", file=sys.stderr)
-        sys.exit(1)
+        projects = ['.',]  # start it in the local project by default
 
     all_projects = self.GetProjects(projects,
                                     missing_ok=bool(self.gitc_manifest))