Fixes git_compare_version() in lib/git.zsh

git_compare_version() is used to determine whether dirty submodules should be ignored or not, but git has to be more recent than version 1.7.2 to know about that option.

With the former comparison logic a check if git version 1.8.1 is more recent than 1.7.2 turned out negative, which is fixed through this pull request.