Merge branch 'jc/resolve-undo' into maint

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