Merge branch 'mp/absorb-submodule-git-dir-upon-deinit'

Merge branch 'mp/absorb-submodule-git-dir-upon-deinit'

"git submodule deinit" for a submodule whose .git metadata
directory is embedded in its working tree refused to work, until
the submodule gets converted to use the "absorbed" form where the
metadata directory is stored in superproject, and a gitfile at the
top-level of the working tree of the submodule points at it.  The
command is taught to convert such submodules to the absorbed form
as needed.

* mp/absorb-submodule-git-dir-upon-deinit:
  submodule: absorb git dir instead of dying on deinit