determinator.rb: Check is git repo only once
determinator.rb: Check is git repo only once

When there is no git repository, `is_git_repo?` was false,
and the `||=` checks for a git repository every invocation.

Fixes https://github.com/gjtorikian/jekyll-last-modified-at/issues/60

gjtorikian/jekyll-last-modified-at,gjtorikian/jekyll-last-modified-at