dagutil: use a listcomp instead of a map()
In Python 3, the map() returns a generator object instead of a list,
and some parts of hg depend on this being consumable more than once or
sortable in place.
Differential Revision: https://phab.mercurial-scm.org/D1099
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.