Merge branch 'jt/pretend-object-never-come-from-elsewhere'

Merge branch 'jt/pretend-object-never-come-from-elsewhere'

The pretend-object mechanism checks if the given object already
exists in the object store before deciding to keep the data
in-core, but the check would have triggered lazy fetching of such
an object from a promissor remote.

* jt/pretend-object-never-come-from-elsewhere:
  sha1-file: make pretend_object_file() not prefetch