rust: start plugging the dirstate tree behind a feature gate...
rust: start plugging the dirstate tree behind a feature gate
The previous patch added the `dirstate-tree` feature gate to enable the two
dirstate implementations to co-habit while the tree-based one gets better.
This patch copies over the code that differs, be it because the algorithm
changed or because the borrowing rules are different.
Indeed, `DirstateTree` is not observationally equivalent to the std `HashMap` in
the APIs we use: it does not have the `Entry` API (yet?) and its iterator
returns owned values instead of references. This last point is because the
implementation needs to be changed to a more clever and efficient solution.
Differential Revision:
https://phab.mercurial-scm.org/D9133
