ObjectWalk: Fix reset for non-commit objects

ObjectWalk: Fix reset for non-commit objects

Non-commits are added to a pending queue, but duplicates are
removed by checking a flag.  During a reset that flag must be
stripped off the old roots, otherwise the caller cannot reuse
the old roots after the reset.

RevWalk already does this correctly for commits, but ObjectWalk
failed to handle the non-commit case itself.

Change-Id: I99e1832bf204eac5a424fdb04f327792e8cded4a
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>