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

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