Running the following two commands:

devtool extract netbase netbase-src
devtool sync netbase netbase-src

results in the following error:

Traceback (most recent call last):
  File "/home/pkj/dists/poky-master/scripts/devtool", line 350, in <module>
    ret = main()
  File "/home/pkj/dists/poky-master/scripts/devtool", line 337, in main
    ret = args.func(args, config, basepath, workspace)
  File "/home/pkj/dists/poky-master/poky/scripts/lib/devtool/standard.py", line 453, in sync
    initial_rev, _ = _extract_source(srctree, args.keep_temp, args.branch, True, config, basepath, workspace, args.fixed_setup, rd, tinfoil, no_overrides=True)
  File "/home/pkj/dists/poky-master/poky/scripts/lib/devtool/standard.py", line 668, in _extract_source
    bb.process.run('git fetch file://' + srcsubdir + ' ' + devbranch + ':' + devbranch, cwd=srctree)
  File "/home/pkj/dists/poky-master/poky/bitbake/lib/bb/process.py", line 189, in run
    raise ExecutionError(cmd, pipe.returncode, stdout, stderr)
bb.process.ExecutionError: Execution of 'git fetch file:///home/pkj/dists/poky-master/builds/qemux86-64/tmp/work/all-poky-linux/netbase/6.4/devtooltmp-gyqm88u7/workdir/netbase devtool:devtool' failed with exit code 128:
fatal: refusing to fetch into branch 'refs/heads/devtool' checked out at '/home/pkj/dists/poky-master/builds/qemux86-64/netbase-src'

The result is the same for a recipe that fetches from Git, e.g.:

devtool extract update-rc.d update-rc.d-src
devtool sync update-rc.d update-rc.d-src