branchmap: add a cache validation cache, avoid expensive re-hash on every use In a pathological `hg log` case, we end up executing the branchmap validity checking twice per commit displayed. Or maybe we always do, and I just noticed because it's really slow in this repo for some reason. Before: ``` Time (mean ± σ): 9.816 s ± 0.071 s [User: 9.435 s, System: 0.392 s] Range (min … max): 9.709 s … 9.920 s ``` After: ``` Time (mean ± σ): 8.671 s ± 0.078 s [User: 8.309 s, System: 0.392 s] Range (min … max): 8.594 s … 8.816 s ``` Differential Revision: https://phab.mercurial-scm.org/D9023