When there are many environments, loading diffs, blobs, and comparisons
can be slow because we load many environments and call CommitIsAncestor
on each of them to determine whether each environment is relevant to a
given commit SHA. This is a bit of a waste because in most cases, the
controller just needs the latest environment for the given commit.

This commit adds a `find_latest` flag to `EnvironmentsFinder` to look
for only one environment that matches the criteria. In the common case,
we will only need to look for one or two environments instead of
iterating through all of them.

Closes https://gitlab.com/gitlab-org/gitlab/issues/29562