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.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present