Remove --quiet from git submodule operations
Add documentation about mutable statics to rust.md
auto merge of #7846 : alexcrichton/rust/static-mut-dox, r=pnkfelix

It's probably a good idea to at least *mention* them somewhere.
auto merge of #7847 : alexcrichton/rust/verbose-submodule, r=pnkfelix

Closes #3816.

Without --quiet, git shows its own progress report of download. It's not really a progress bar, but it's a percentage and files incoming. This will help initial downloads of LLVM to not cause people to wonder why their configure script is hanging for hours.

r? @graydon