Add support for branch option in git gems
Simplify test_gem_git

There's no reason to try to create or load git gems.
Use master as the default git gem reference

This matches bundler behavior.
Remove default reference argument

This should be given by all callers and only defaulted in a gem.deps.rb.
Add git gem tag support
Implement git gem ref support
Test git gem reference preference order
Refresh git gem cache when it already exists

This avoids git errors.
Test reference support or git gem sources