gerrit: correct the field name for commit message in RevisionInfo

The actual field returned by Gerrit in practice is called
CommitWithFooters:
https://gerrit.googlesource.com/gerrit/+/b2ce0c1008bb2755b999f45bbf4c81b8bb3aaaaf/java/com/google/gerrit/extensions/common/RevisionInfo.java

The field was apparently called "message with footers" here because of
out-of-date or inaccurate documentation; docs updated in
https://gerrit-review.googlesource.com/c/gerrit/+/227372.

Change-Id: I03c6ecda9eb49f12250c044d16f10b78c56c3925
Reviewed-on: https://go-review.googlesource.com/c/build/+/181340
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: Quinten Yearsley

gerrit/gerrit.go

@@ -341,7 +341,7 @@ type RevisionInfo struct {
 	Fetch             map[string]*FetchInfo `json:"fetch"`
 	Commit            *CommitInfo           `json:"commit"`
 	Files             map[string]*FileInfo  `json:"files"`
-	MessageWithFooter string                `json:"messageWithFooter"`
+	CommitWithFooters string                `json:"commit_with_footers"`
 	Kind              string                `json:"kind"`
 	// TODO: more
 }