tests/test-check-rust-format.t

rust: Add a Timestamp struct instead of abusing Duration

`SystemTime` would be the standard library type semantically appropriate
instead of `Duration`.

But since the value is coming from Python as a plain integer and used in
dirstate packing code as an integer, let’s make a type that contains a single
integer instead of using one with sub-second precision.

Differential Revision: https://phab.mercurial-scm.org/D10485

#require rustfmt test-repo

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

  $ cd "$TESTDIR"/..
  $ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
  $ for f in `testrepohg files 'glob:**/*.rs'` ; do
  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
  > done