Replace git_mirror.pl with refactored, more robust git wrapper

Our git wrapper is quite naive and fragile, and effectively unable to be extended to cover our new needs (such as modularization's need for git submodules).

We need to replace it with a new version which is refactored, covered by autotests, and handles a larger set of git commands. At least we must handle:

• git clone
• git fetch
• git pull
• git submodule

This would be used to fix the current issue we have that modularization takes 3 hours cloning webkit for each test.