dirs: reject consecutive slashes in paths 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 slashes. Differential Revision: https://phab.mercurial-scm.org/D7252