Merge branch 'hv/submodule-alt-odb'

When peeking into object stores of submodules, the code forgot that they
might borrow objects from alternate object stores on their own.

By Heiko Voigt
* hv/submodule-alt-odb:
  teach add_submodule_odb() to look for alternates

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc submodule.c
Simple merge