store: cache the file_size when we get it from disk
The point of caching `files` is to ensure consistency and avoiding redoing
expensive work. So we cache the file_size once retrieved.
https://bz.mercurial-scm.org/1089
$ hg init repo
$ cd repo
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..