perf-discovery: use `get_unique_pull_path`
This performance command now use the new API, unless the benchmarked Mercurial
is older.
Differential Revision: https://phab.mercurial-scm.org/D10423
#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