Replace dedicated Commit Field with an existing one

Summary: fix T15600

Test Plan: arc diff against this

Reviewers: Sten, O1 Blessed Committers!

Reviewed By: Sten

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15600

Differential Revision: https://we.phorge.it/D25385