Merge branch 'jc/resolve-undo' into maint

The resolve-undo information in the index was not protected against GC, which has been corrected. source: <xmqq35f7kzad.fsf@gitster.g> * jc/resolve-undo: fsck: do not dereference NULL while checking resolve-undo data revision: mark blobs needed for resolve-undo as reachable