devtool: standard: throws appropriate error if source is in detached HEAD

If source is in detached HEAD, we get the following error when using
detvool finish command:

    [...]
    File "<...>/poky/scripts/lib/devtool/standard.py", line 1938, in _update_recipe
      bb.process.run('git checkout %s' % startbranch, cwd=srctree)
    File "<...>/poky/bitbake/lib/bb/process.py", line 189, in run
      raise ExecutionError(cmd, pipe.returncode, stdout, stderr)
  bb.process.ExecutionError: Execution of 'git checkout (HEAD detached at 9bbf87e)' failed with exit code 2:
  /bin/sh: -c: line 1: syntax error near unexpected token `('
  /bin/sh: -c: line 1: `git checkout (HEAD detached at 9bbf87e)'

Check this and throws an appropriate error in this case

(From OE-Core rev: d9c686b5ff9f591ec6b928ed539084c02df4c8a5)

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/devtool/standard.py

@@ -1885,6 +1885,8 @@ def _update_recipe(recipename, workspace, rd, mode, appendlayerdir, wildcard_ver
         for line in stdout.splitlines():
             branchname = line[2:]
             if line.startswith('* '):
+                if 'HEAD' in line:
+                    raise DevtoolError('Detached HEAD - please check out a branch, e.g., "devtool"')
                 startbranch = branchname
             if branchname.startswith(override_branch_prefix):
                 override_branches.append(branchname)