revision: ensure full index

revision: ensure full index

Before iterating over all index entries, ensure that a sparse index is
expanded to a full index to avoid unexpected behavior. This case could
be integrated later by ensuring that we walk the tree in the
sparse-directory entry, but the current behavior is only expecting
blobs. Save this integration for later when it can be properly tested.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>