Jeff King authored Jul 13, 2014 and Junio C Hamano committed Jul 28, 2014

The point of the "index" field of struct commit is that
every allocated commit would have one. It is supposed to be
an invariant that whenever object->type is set to
OBJ_COMMIT, we have a unique index.

Commit 969eba63

 (commit: push commit_index update into
alloc_commit_node, 2014-06-10) covered this case for
newly-allocated commits. However, we may also allocate an
"unknown" object via lookup_unknown_object, and only later
convert it to a commit. We must make sure that we set the
commit index when we switch the type field.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>