Merge branch 'py/diff-submodule'

* py/diff-submodule:
is_racy_timestamp(): do not check timestamp for gitlinks
diff-lib.c: rename check_work_tree_entity()
diff: a submodule not checked out is not modified
Add t7506 to test submodule related functions for git-status
t4027: test diff for submodule with empty directory

diff --cc diff-lib.c
Simple merge

diff --cc read-cache.c
Simple merge