shortest: never emit 0-length prefix even if unique It turned out that the pure version of our code for finding the shortest unique nodeid prefix would return a 0-length string if that was unique (because there was at most one revision in the disambiguation set). That's kind of correct, but it can't be used as input, so we shouldn't return it. Let's just adjust the given minlength up to at least 1. This fixes test-template-functions.t, which was failing in pure mode. Differential Revision: https://phab.mercurial-scm.org/D5181