Support ignore.txt deletions with supershipit

[mattclay]

PRs can be auto-merged when they contain deletions to sanity ignore files. However, it appears this feature cannot be used with supershipit privileges.

See the existing support for sanity ignore files:

ansibullbot/ansibullbot/triagers/plugins/shipit.py

Lines 113 to 119 in a6e389c

	elif fnmatch(thisfn, u'test/sanity/*/*.txt'):
	if pr_file.additions or pr_file.status == u'added':
	# new exception added, addition must be checked by an human
	return create_ameta(False, u'automerge new file(s) test failed')
	if pr_file.deletions:
	# new exception delete
	continue

Here's a PR which cannot be merged with supershipit privileges: ansible/ansible#59324

[jctanner]

resolved by #1275