Merge branch 'sb/submodule-short-status' into next

Merge branch 'sb/submodule-short-status' into next

The output from "git status --short" has been extended to show
various kinds of dirtyness in submodules differently; instead of to
"M" for modified, 'm' and '?' can be shown to signal changes only
to the working tree of the submodule but not the commit that is
checked out.

* sb/submodule-short-status:
  submodule.c: correctly handle nested submodules in is_submodule_modified
  short status: improve reporting for submodule changes
  submodule.c: stricter checking for submodules in is_submodule_modified
  submodule.c: port is_submodule_modified to use porcelain 2
  submodule.c: convert is_submodule_modified to use strbuf_getwholeline
  submodule.c: factor out early loop termination in is_submodule_modified
  submodule.c: use argv_array in is_submodule_modified