Avoid hitting statement timeout finding MR pipelines39054-activerecord-statementinvalid-pg-querycanceled-error-canceling-statement-due-to-statement-timeout

For MRs with many thousands of commits, `SELECT DISTINCT(sha)` will be very
slow.

What we can't do to fix this:

1. Add an index. Postgres won't use it for DISTINCT without a lot of ceremony.
2. Do the `uniq` in Ruby. That can still be very slow with hundreds of
   thousands of commits.
3. Use a subquery. We haven't removed the `st_commits` column yet, but we will
   soon.

Until 3 is available to us, we can just do 2, but also add a limit clause. There
is no ordering, so this may return different results, but our goal with these
MRs is just to get them to load, so it's not a huge deal.

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