Merge branch 'jt/avoid-prefetch-when-able-in-diff'

Merge branch 'jt/avoid-prefetch-when-able-in-diff'

"git diff" in a partial clone learned to avoid lazy loading blob
objects in more casese when they are not needed.

* jt/avoid-prefetch-when-able-in-diff:
  diff: restrict when prefetching occurs
  diff: refactor object read
  diff: make diff_populate_filespec_options struct
  promisor-remote: accept 0 as oid_nr in function