Mark an object or commit to start graph traversal from.
Callers are encouraged to use {@link RevWalk#parseAny(AnyObjectId)}instead of {@link RevWalk#lookupAny(AnyObjectId,int)}, as this method requires the object to be parsed before it can be added as a root for the traversal.
The method will automatically parse an unparsed object, but error handling may be more difficult for the application to explain why a RevObject is not actually valid. The object pool of this walker would also be 'poisoned' by the invalid RevObject.
This method will automatically call {@link RevWalk#markStart(RevCommit)}if passed RevCommit instance, or a RevTag that directly (or indirectly) references a RevCommit.
@param o the object to start traversing from. The object passed must be from this same revision walker.
@throws MissingObjectException the object supplied is not available from the object database. This usually indicates the supplied object is invalid, but the reference was constructed during an earlier invocation to {@link RevWalk#lookupAny(AnyObjectId,int)}.
@throws IncorrectObjectTypeException the object was not parsed yet and it was discovered during parsing that it is not actually the type of the instance passed in. This usually indicates the caller used the wrong type in a {@link RevWalk#lookupAny(AnyObjectId,int)} call.
@throws IOException a pack file or loose object could not be read.