rhg: use a release-mode executable in tests
This allows the rhg build for test-rhg.t to share compiled dependencies
such as hg-core with the hg-cpython build for other tests.
For context, my wrapper script for the typical edit-compile-test
cycle now looks like this:
(cd rust && cargo +nightly-2020-10-04 fmt)
(cd rust && cargo build --release -p rhg)
make --silent local PURE=--rust
python test/run-tests.py --local "$@"
Differential Revision: https://phab.mercurial-scm.org/D9728
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=$PYTHON "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF