rust-matchers: implement `visit_children_set` for `FileMatcher`
As per the removed inline comment, this will become useful in a future patch
in this series as the `IncludeMatcher` is introduced.
Differential Revision: https://phab.mercurial-scm.org/D7914
Ancestors of 5
4 2 0
Ancestors of 6 and 5
4 3 2 1 0
Ancestors of 5 and 4
4 2 0
Ancestors of 7, stop at 6
6
Ancestors of 7, including revs
7 6 5 4 3 2 1 0
Ancestors of 7, 5 and 3, including revs
7 6 5 4 3 2 1 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8