Merge branch 'bw/submodule-branch-dot-doc' into maint

Recent git allows submodule.<name>.branch to use a special token
"." instead of the branch name; the documentation has been updated
to describe it.

* bw/submodule-branch-dot-doc:
  submodules doc: update documentation for "." used for submodule branches

diff --cc Documentation/gitmodules.txt
Simple merge