.gitattributes

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%)

# So GitLab doesn't think we're using tons of Perl
*.t -linguist-detectable