Commit write performance improvement

[miloszm]

Summary

Commit command returns commit which is used only for root calculation, this is sub-optimal as it increases commit persistence time by ca 20% because of extra commit clone and extra retrieval which can be avoided. Similarly, when storing index, converting its serialised form to vector is not needed and also costs ca 15% of time.

Possible solution design or implementation

From Commit command, pass back hash rather than entire commit, do not retrieve commit, and do not convert serialised index to an extra vector.