R1:29fd3f136b88

Allow columns to be marked as nonmutable (so save() will not change them)

Summary:
Ref T6840. This feels a little dirty; open to alternate suggestions.

We currently have a race condition where multiple daemons may load a commit and then save it at the same time, when processing "reverts X" text. Prior to this feature, two daemons would never load a commit at the same time.

The "reverts X" load/save has no effect (doesn't change any object properties), but it will set the state back to the loaded state…