rust-dirstate: panic if the DirstateMap counters go below 0 When modifying the API I hit some... interesting errors (trying to allocate 178GB of RAM, for example) because I failed to keep the counters correctly updated. This counter underflow is likely to happen when code is changed around and can have up to eat-your-dirstate level of consequences, which is not nice. The very small runtime cost of checking these counters should really not be an issue and will help us uncover bugs when/if they do appear in the future. Differential Revision: https://phab.mercurial-scm.org/D12430