rust-index: use the new method in shortesthexnodeidprefix
This code can now run with both a Rust or a C index.
Differential Revision: https://phab.mercurial-scm.org/D7659
#require emacs
$ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
> -f ert-run-tests-batch-and-exit
Running 1 tests (*) (glob)
passed 1/1 hg-test-mode--compilation-mode-support
Ran 1 tests, 1 results as expected (*) (glob)