shortest: make {shortest("fffffffff")} work again {shortest("fffffffff")} should shorten it to the shortest unambiguous prefix for the working directory. It used to do that until I broke it in 7b2955624777 (scmutil: make shortesthexnodeidprefix() take a full binary nodeid, 2018-04-14), when we started returning the full hex nodeid for any working directory prefix shorter than 40 hex digits. This patch fixes it by catching WdirUnsupported specifically. Differential Revision: https://phab.mercurial-scm.org/D3455