localrepo: cache types for filtered repos (issue5043)
Python introduces a reference cycle on dynamically created types via __mro__, making them very easy to leak. See https://bugs.python.org/issue17950. Previously, repo.filtered() created a type on every invocation. Long-running processes (like `hg convert`) could call this function thousands of times, leading to a steady memory leak. Since we're Unable to stop the leak because this is a bug in Python, the next best thing is to contain it. This patch adds a cache of of the dynamically generated repoview/filter types on the localrepo object. Since we only generate each type once, we cap the amount of memory that can leak to something reasonable. After this change, `hg convert` no longer leaks memory on every revision. The process will likely grow memory usage over time due to e.g. larger manifests. But there are no leaks.
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mentioned in commit d100d15491ef
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mentioned in commit 46db86edf8f2
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mentioned in commit 76ae43d5b1db
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