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

diff --cc cache.h
Simple merge

diff --cc diff.c
Simple merge