tests/test-check-rust-format.t

dirstate: explicitly write the dirstate after `keyword` "overwrite"

I am working on making the dirstate write patterns more predictable. This patch is
part of a small series of similar patches that adds a explicit dirstate write in
a handful of location where the dirstate is updated "a bit in a strange way".

With this explicit write, we are no longer relying on implicite write of the
dirstate on `wlock` release. This make the world a better place.

#require rustfmt test-repo

  $ . "$TESTDIR/helpers-testrepo.sh"

  $ cd "$TESTDIR"/..

Warning: Keep this in sync with hghave.py
  $ RUSTFMT=$(rustup which --toolchain nightly-2021-11-02 rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done