We shouldn't ever see those, and the fuzzer go really excited that if
it gives us a 65k string with 55k slashes in it we use a lot of RAM.
This is a better fix than what I tried in D7105. It was suggested by
Yuya, and I verified it does in fact cause the fuzzer to not OOM.
This is a revision of D7234, but with the missing set of an error
added. I added a unit test of the dirs behavior because I needed to
reason more carefully about the failure modes around consecutive