context: don't use util.cachefunc due to cycle creation (issue5043) util.cachefunc stores all arguments as the cache key. For filectxfn functions, the arguments include the memctx instance. This creates a cycle where memctx._filectxfn references self. This causes a memory leak. We break the cycle by implementing our own memoizing function that only uses the path as the cache key. Since each memctx has its own cache instance, there is no concern about invalid cache hits.