shortest: make pure code also disambigute against revnums at end This makes the pure code more similar to the native code in that it first finds a prefix that's unambiguous among nodeids and then adds hex digits until it no longer looks like a revnum. It will allow us to even better separate the disambiguation with revnums in a later patch. With this patch `hg log -r 0::50 -T '{shortest(node,1)}'` with no native code goes from 25s to 43s. It wasn't exactly usable to begin with, so I don't feel too bad about it. Differential Revision: https://phab.mercurial-scm.org/D3500