ashleygwilliams/cargo-generate

fix relative paths to templates - DD5HT, issue/128 pull/129

When we previously merged the PR in 0.2.0 that leveraged cargo to clone the templates,
enabling folks to work with private repositories- we introduced a GitConfig::new function
(replacing work done previously by libgit2). This function works great- but did not support
relative paths. We didn't catch this because we weren't testing the relative paths usecase!

With this PR, DD5HT has restored the relative path functionality- and added a test to prevent
further regressions of this function!