dgit: cleaning: Improve handling of note about ignores

dgit: cleaning: Improve handling of note about ignores

* Provide a new $ignmessage parameter to clean_tree_check_git and use
  that.  This makes it a bit easier to:

* Only print the additional message if the clean mode actually
  honoured ignores.  If it didn't then the problem is not `due
  to missing .gitignore entries' because they would not have helped.

* Change all callers other than clean_tree_check_git_wd to
  pass the empty string, so no other overall functional change.

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