Use a separate feature flag for partitioned commits table reads

Eugenia Grieffrequested to merge
600540-change-ff-for-commits-table-pruning into master

What does this MR do and why?

Move commits partitioned table reads to a new feature flag guard

Decouple reading and writing to the new diff commits table by introducing a separate FF mr_diff_commits_read_new_table to guard the read paths.

Writing paths remain guarded by the FF merge_request_diff_commits_partition.

References

Related to #600540

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