store: change `_walk` return to `(filename, (type, size))`
If we are to group file per revlog, having the filename as the "main key" will be useful. This change will make the following changes clearer.
%include map-cmdline.default
[templates]
changeset = '{cset}{branches}{bookmarks}{tags}{lphase}{parents}{luser}{ldate}{summary}\n'
changeset_verbose = '{cset}{branches}{bookmarks}{tags}{lphase}{parents}{luser}{ldate}{lfiles}{lfile_copies_switch}{description}\n'