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.
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