rust-dirstate-v2: save proper data size if no new data on append
This is currently only triggered with the tests ran with `--rhg` without
`--rust`, by "luck", there probably always was something to write, like an
mtime when also using Rust extensions alongside `rhg`.
Differential Revision: https://phab.mercurial-scm.org/D12580
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true