revset: define successors revset
This revset returns all successors, including transit nodes and the source
nodes (to be consistent with existing revsets like "ancestors").
To filter out transit nodes, use `successors(X)-obsolete()`.
To filter out divergent case, use `successors(X)-divergent()-obsolete()`.
The revset could be useful to define rebase destination, like:
`max(successors(BASE)-divergent()-obsolete())`. The `max` is to deal with
splits.
There are other implementations where `successors` returns just one level of
successors, and `allsuccessors` returns everything. I think `successors`
returning all successors by default is more user friendly. We have seen
cases in production where people use 1-level `successors` while they really
want `allsuccessors`. So it seems better to just have one single revset
returning all successors by default to avoid user errors.
In the future we might want to add `depth` keyword argument to it and for
other revsets like `ancestors` etc. Or even build some flexible indexing
syntax [1] to satisfy people having the depth limit requirement.
[1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-July/101140.html
$ 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