rust-index: avoid some cloning in find_gca_candidates()
Instead of keeping the information whether the current revision is
poisoned on `current_seen`, we extract it as a boolean.
This also allows us to simplify the explanation of `seen[r].is_poisoned()`,
as the exceptional case where it is poisoned right after `r` has been
determined to be a solution does no longer exist.
#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* (glob)
Ran 1 tests, 1 results as expected* (glob)