Merge branch 'jt/cache-tree-avoid-lazy-fetch-during-merge'

Merge branch 'jt/cache-tree-avoid-lazy-fetch-during-merge'

The cache-tree code has been taught to be less aggressive in
attempting to see if a tree object it computed already exists in
the repository.

* jt/cache-tree-avoid-lazy-fetch-during-merge:
  cache-tree: do not lazy-fetch tentative tree