generatorset: make it possible to use gen as fastasc or fastdesc
We gain a parameter to inform that the generator is ascending or descending. If
the generator is ordered, it is also used for the `fastasc` or `fastdesc`
version.
The _ascgeneratorset and _descgeneratorset class will be removed soon.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a