Merge branch 'jk/diff-no-rename-empty'

Merge branch 'jk/diff-no-rename-empty'

Forbids rename detection logic from matching two empty files as renames
during merge-recursive to prevent mismerges.

By Jeff King
* jk/diff-no-rename-empty:
  merge-recursive: don't detect renames of empty files
  teach diffcore-rename to optionally ignore empty content
  make is_empty_blob_sha1 available everywhere
  drop casts from users EMPTY_TREE_SHA1_BIN