Merge branch 'jl/submodule-add-by-name'

Merge branch 'jl/submodule-add-by-name'

If you remove a submodule, in order to keep the repository so that
"git checkout" to an older commit in the superproject history can
resurrect the submodule, the real repository will stay in $GIT_DIR
of the superproject.  A later "git submodule add $path" to add a
different submodule at the same path will fail.  Diagnose this case
a bit better, and if the user really wants to add an unrelated
submodule at the same path, give the "--name" option to give it a
place in $GIT_DIR of the superproject that does not conflict with
the original submodule.

* jl/submodule-add-by-name:
  submodule add: Fail when .git/modules/<name> already exists unless forced
  Teach "git submodule add" the --name option