Echo the 'git submodule update' command that will be run when updating a submodule.  This will generally only happen for developers who are actually in a git clone (not for users installing from a source tarball).  This should help reduce surprise for developers working on a submodule
Integrate better with the warning messages that are already there
Merge pull request #62 from embray/echo-git-submodule

Explicitly show all git commands that are run
Prevent git from fetching/cloning from the internet when the --offline flag is used