repo: avoid copying/updating a dict on every `repo.__getitem__` This has some mild performance benefits. I'm looking into a pathological case where one of our `hg log` invocations takes several seconds, and according to hyperfine this reduces the wall time of the entire operation (running in chg) from: ``` Time (mean ± σ): 7.390 s ± 0.106 s [User: 7.058 s, System: 0.271 s] Range (min … max): 7.300 s … 7.625 s ``` to: ``` Time (mean ± σ): 7.046 s ± 0.091 s [User: 6.714 s, System: 0.279 s] Range (min … max): 6.916 s … 7.169 s ``` Note: the log command is slow due to an issue in our custom stuff executing `repo[<arg>]` 298,800 times. This performance improvement is likely not noticeable during normal operation, but I don't feel like it's making the code more difficult to understand, and every small bit helps. Differential Revision: https://phab.mercurial-scm.org/D9022