revset: added lazyset implementation to desc revset
Performance benchmarking:
$ time hg log -qr "first(desc(hg))"
changeset: 0:
9117c6561b0b
real 0m2.210s
user 0m2.158s
sys 0m0.049s
$ time ./hg log -qr "first(desc(hg))"
changeset: 0:
9117c6561b0b
real 0m0.171s
user 0m0.131s
sys 0m0.035s
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.