D5480: tests: show that debugrebuilddirstate is broken with narrow+sparse

[Yuya Nishihara]

> +  $ hg debugrebuilddirstate
> +  ** unknown exception encountered, please report by visiting
> +  ** https://mercurial-scm.org/wiki/BugTracker
> +  ** Python 2.7.12 (default, Nov 12 2018, 14:36:49) [GCC 5.4.0 20160609]
> +  ** Mercurial Distributed SCM (version 4.8.1+588-479a5ea51ccc+20181224)
> +  ** Extensions loaded: narrow, sparse
> +  Traceback (most recent call last):
> +    File "/place/vartmp/hgtests.zMelCK/install/bin/hg", line 43, in <module>
> +      dispatch.run()

Next time, please don't include a raw traceback, which makes the test
unstable.