Rename detection logic used to match two empty files as renames during
merge-recursive, leading unnatural 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