We are currently keeping around commits of each note being
published even though they are all the same commits. That results
to duplicate `ref_exists` calls.

For example, if there are 6 draft notes set to be published on a
single MR version, there will be around 36 calls for `ref_exists`.

This fix reduces those calls by getting the unique shas to be kept.
Based on local testing, this reduced the `ref_exists` calls from
36 to 4.

Changelog: performance