wcsfilecmp: make sure trailing slashes sort first

This command

	cd $(mktemp -d)
	mkdir a "a b"
	complete -C": "

prints

	a b/
	a/

which is wrong ordering.
Usually the trailing slash should not be compared.

Fix this by always sorting slashes first.  Not sure if this is correct
for middle slashes but I couldn't find a case where it matters.

Closes #12695