Remove --quiet from git submodule operations
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