In a partial clone that will lazily be hydrated from the
originating repository, we generally want to avoid "does this
object exist (locally)?" on objects that we deliberately omitted
when we created the clone.  The cache-tree codepath (which is used
to write a tree object out of the index) however insisted that the
object exists, even for paths that are outside of the partial
checkout area.  The code has been updated to avoid such a check.

* jt/cache-tree-allow-missing-object-in-partial-clone:
  cache-tree: skip some blob checks in partial clone