Merge branch 'sb/mv-submodule-fix' into HEAD

"git mv old new" did not adjust the path for a submodule that lives
as a subdirectory inside old/ directory correctly.

* sb/mv-submodule-fix:
  mv: allow moving nested submodules

diff --cc builtin/mv.c
Simple merge

diff --cc t/t7001-mv.sh
Simple merge