Previously #get(RevCommit, PatchSet.Id) would NPE if the body of the
commit was not parsed; callers needed to remember to either parse the
body or ensure that the RevWalk that created the commit had
setRetainBody(true). In all cases, there is already a handy RevWalk;
pass this into get and do the parseBody there, such that it's
impossible for the caller to forget. (In the BatchUpdate.Op cases, we
need to shuffle call order around a bit so the PatchSetInfo gets
populated during updateRepo.)
Change-Id: I2e42ed1cc5c4a50958b6367051065e6bbad5cc54
b9805aee6d