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