rust/hg-core/rustfmt.toml

tests: replace `tac` reimplementation by `sort -r`

`sort -r` is better code "formatter" than `tac` since it's
stable. It's also portable so we don't need to reimplement it in
Python.

Differential Revision: https://phab.mercurial-scm.org/D5239

max_width = 79
wrap_comments = true
error_on_line_overflow = true