commit-graph: writing missing parents is a BUG

[derrickstolee]

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.

[kewillford]

There were a couple of other places that GRAPH_PARENT_MISSING was being used.

	if (count_distinct >= GRAPH_PARENT_MISSING)
		die(_("the commit graph format cannot write %d commits"), count_distinct);

	if (commits.nr >= GRAPH_PARENT_MISSING)
		die(_("too many commits to write graph"));

Should GRAPH_PARENT_MISSING be renamed to something like MAX_NUMBER_OF_COMMITS now?

[kewillford]

Approve with suggestion.