commit-reach: reject cycles in contains walk

commit-reach: reject cycles in contains walk

The memoized contains traversal used by git tag assumes that commit
ancestry is acyclic. Replacement refs can violate that assumption,
causing it to keep pushing an already active commit until memory is
exhausted.

Mark commits while they are active and die if the traversal encounters
an active commit. Other failures in this walk already die through
parse_commit_or_die(); using a second reachability walk would only add
a separate policy for malformed history.

Suggested-by: Kristofer Karlsson <redacted>
Signed-off-by: Tamir Duberstein <redacted>
Signed-off-by: Junio C Hamano <redacted>