tidy: prohibit FIXME without an issue number

[reidpr]

Split from #1860. Pylint: W0511 fixme

charliecloud/lib/build_cache.py

Lines 598 to 601 in d679373

	def update(self, path):
	"Recompute the File_Metadata object at path in the tree rooted by me."
	# FIXME: Can’t handle anything other than regular, non-large files that
	# don’t need renaming.

Just in source code this is lost. Require such comments to point to an issue number.