view CONTRIBUTING @ 45610:496537c9c1b4

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

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).