store: directly pass the filesize in the `details` of revlog
The dictionary only contains 1 (or 0) entries, we can directly store that
information (or None).
Moving to a simpler argument passing result in a noticable speedup (because
Python)
The number below use `hg perf::stream-locked-section` to measure the time spend
in the locked section of the streaming clone. Number are run on various
repository.
### mercurial-2018-08-01-zstd-sparse-revlog
before: 0.031247 seconds
after: 0.030246 seconds (-3.20%)
### mozilla-central-2018-08-01-zstd-sparse-revlog
before: 6.718968 seconds
after: 6.304179 seconds (-6.17%)
### mozilla-try-2019-02-18-zstd-sparse-revlog
before: 14.631343 seconds
after: 14.142687 seconds (-3.34%)
### netbeans-2018-08-01-zstd-sparse-revlog
before: 2.895584 seconds
after: 2.719939 seconds (-6.07%)
### pypy-2018-08-01-zstd-sparse-revlog
before: 0.561843 seconds
after: 0.543034 seconds (-3.35%)
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