Silently permit invalid ObjectIds during RefUpdate
If we try to parse objects which don't exist it may be because
we are trying to force deletion of a ref exactly because the
object the ref currently refers to has been pruned from the
object database already. Rather than fail with an exception
we should still permit the removal.
This also fixes the breakage in the unit tests introduced by
Charles'
5a318367 "Refactor of RefUpdate force ...". One of
the unit tests for RefUpdate is using a bad ObjectId as the
new value for the ref, and uses forceUpdate to hammer it into
the ref database anyway, even though it is not valid as the
object does not exist.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>