changegroup: sparsely populate fnodes Previously, fnodes had a key and empty dict value for every element in changedfiles. This is somewhat wasteful. Empty dicts in CPython consume a lot more memory than you would expect - 280 bytes. On mozilla-central, which has ~190,000 files/fnodes keys, the previous loop populating fnodes allocated 91,924 KB of memory, most of that for the empty dicts. With this patch in place, our peak RSS during mozilla-central clone drops: before: 364,356 KB after: 326,008 KB delta: -38,348 KB When combined with the previous patch, total peak RSS decrease is now 190,116 KB.