Revert "Performance: lazily memoize expensive `prerelease?` values during resolve"

Too soon! This is part of changes in the master branch for 1.8.

This reverts commit ed8ad7dac4f89d283c1e7a4c0662cd88ce75e613.
CI fixes backported from master
Fix gem_helper_spec.rb when run with git 2.0.

Git now errors out if you haven't set a default push behavior.
Fix comparison of frozen sources.