discovery: fix docstring of `outgoing` class
Also, introduce a more correct name `ancestorsof` for what was named
`missingheads` before. For now, we just forward `ancestorsof` to `missingheads`
until all users are changed.
There were some mistakes in the old docstring / name:
* `missingheads` (new name: `ancestorsof`) contains the revs whose ancestors
are included in the outgoing operation. It may contain non-head revs and revs
which are already on the remote, so the name "missingheads" is wrong in two
ways.
* `missing` contains only ancestors of `missingheads`, so not *all nodes*
present in local but not in remote.
* `common` might not contain all common revs, e.g. not some that are not an
ancestor of `missingheads`.
It seems like the misleading name have fostered an actual bug (issue6372),
where `outgoing.missingheads` was used assuming that it contains the heads of
the missing changesets.
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3