Use an enum for recording which commit fields to compute

Hard-coding the strings always felt ugly, and this is not much more
code. Also, we want to use this data structure for JSON data as well,
so the key names should be decoupled from the idea of keys in a Soy
data map.

Change-Id: Ibe70f1f370783c11c6021531ccae9371f3b899ec
3 files changed