This flag was not being honored due to a bug in createWalk().
argWalk is always non-null when there are commits passed in
on the command line. If --objects was specified, always make
a new ObjectWalk for the actual execution.
Change-Id: I6e1a1636f2634605d86671a83766cc1c42939821
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>