Merge branch 'xx/diff-tree-remerge-diff-fix'

"git rev-list ... | git diff-tree -p --remerge-diff --stdin" should
behave more or less like "git log -p --remerge-diff" but instead it
crashed, forgetting to prepare a temporary object store needed.

* xx/diff-tree-remerge-diff-fix:
  diff-tree: fix crash when used with --remerge-diff

diff --cc builtin/diff-tree.c
Simple merge