dgit: gitattributes: Defuse working-tree-encoding

* Add -working-tree-encoding to $negate_harmful_gitattrs.

* Add new arrangements for updating an existing dgit-defuse-attrs
  macro definition:
    - is_gitattrs: say whether the macro is up to date
    - setup_gitattrs: update an existing out-of-date macro
    - minor message changes
    - document the new behaviour

One consequence is that we have had to breach the promise about how to
stop future dgit setup-new-tree runs messing with this: it is now
necessary to disapply the macro, rather than redefine it.

Closes:#901900.

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