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
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors