Add retainOnReset(RevFlag) to RevWalk to simplify reset usage

Add retainOnReset(RevFlag) to RevWalk to simplify reset usage

Applications sometimes use a RevFlag instead of a Set<RevObject>
to track boolean state bits about objects being processed. However
this requires careful use of the resetRetain() methods to avoid an
accidental clearing of the RevFlag bits, effectively clearing the
Set<RevObject> the application wanted to track.

Simplify that use case by offering retainOnReset, a collection of
flags that are never cleared by the RevWalk.

Change-Id: I4c05b89b1398e4a4f371eac3a5d1d5edddec838f