dirstate-tree: Add `NodeRef` and `ChildNodesRef` enums
They are used instead of `&Node` and `&ChildNodes` respectively.
The `ChildNodes` type alias also becomes a similar enum.
For now they only have one variant each, to be extended later.
Adding enums now forces various use sites go through new methods
instead of manipulating the underlying data structure directly.
Differential Revision: https://phab.mercurial-scm.org/D10747
$ hg init r1
$ cd r1
$ hg ci --config ui.allowemptycommit=true -m c0
$ hg ci --config ui.allowemptycommit=true -m c1
$ hg ci --config ui.allowemptycommit=true -m c2
$ hg co -q 0
$ hg ci --config ui.allowemptycommit=true -m c3
created new head
$ hg co -q 3
$ hg merge --quiet
$ hg ci --config ui.allowemptycommit=true -m c4
$ hg log -G -T'{desc}'
@ c4
|\
| o c3
| |
o | c2
| |
o | c1
|/
o c0
>>> from mercurial import hg
>>> from mercurial import ui as uimod
>>> repo = hg.repository(uimod.ui())
>>> for anc in repo.changelog.ancestors([4], inclusive=True):
... print(anc)
4
3
2
1
0