This field has been nullable since the very beginning (I74ef9f34), but
honestly I have no idea why. Certainly the vast majority of callers
today assume that it's not null.
Force callers to provide a non-null commitId at construction time, and
disallow mutating it later. There was one stubborn caller in
PatchScriptFactory which I couldn't get rid of without disentangling the
logic of the whole class, so I punted and left a deprecated ugly named
static factory method just for this one case.
Change-Id: If8253781c101cbe4ed18210ec9ebe78dee8f4add
03fb740e71