dgit: Do not fail when run with detached HEAD. Closes:#853022.

dgit: Do not fail when run with detached HEAD.  Closes:#853022.

Specifically:

* Pass -q to git-symbolic-ref.  That means that it doesn't print
  an error message when HEAD is not a symbolic ref (ie, a
  detached head), and it means that the exit status is then 1
  rather than 128.

* If the return value from cmdoutput_errok is undef, check $? (which
  is the exit status <<8) and then simply pass on the undef.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>