commit-graph: writing missing parents is a BUG

commit-graph: writing missing parents is a BUG

When writing a commit-graph, we write GRAPH_MISSING_PARENT if the
parent's object id does not appear in the list of commits to be
written into the commit-graph. This was done as the initial design
allowed commits to have missing parents, but the final version
requires the commit-graph to be closed under reachability. Thus,
this GRAPH_MISSING_PARENT value should never be written.

However, there are reasons why it could be written! These range
from a bug in the reachable-closure code to a memory error causing
the binary search into the list of object ids to fail. In either
case, we should fail fast and avoid writing the commit-graph file
with bad data.

Remove the GRAPH_MISSING_PARENT constant in favor of the constant
GRAPH_EDGE_LAST_MASK, which has the same value.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>