ObjectWalk: make setRetainBody(false) the default

ObjectWalk: make setRetainBody(false) the default

Despite being the primary author of RevWalk and ObjectWalk I still
fail to remember to setRetainBody(false) in application code using
an ObjectWalk to examine the graph.

Document the default for RevWalk is setRetainBody(true), where the
application usually wants the commit bodies to display or inspect.

Change the default for ObjectWalk to setRetainBody(false), as nearly
all callers want only the graph shape and do not need the larger text
inside a commit body. This allows some code in JGit to be simplified.

Change-Id: I367e42209e805bd5e1f41b4072aeb2fa98ec9d99