Take an ObjectReader rather than RevWalk. Since validCommit is called
from within a revwalk loop, the constraints on RevWalk --one can't
call next() from within validCommit-- are clearer this way.
Pass RevCommit rather than ObjectId to validCommit.
Don't call rw.ParseBody(commit) in validCommit, because
CommitReceivedEvent already calls parseBody.
Have the caller supply isMerged, so validCommit() does not inspect
magicBranch anymore.
Change-Id: I3b101da8c89f9041ee0ed22e7cdc06cc63277fb5
0f5ff8b1b9