Use git-log instead of git-blame-tree

> This relies on a standard git command to reduce special dependencies.

The `git-blame-tree` command isn't in git, and so people interested in using goblet need to compile their own version of git. This replaces it with `git-log`, which still should be suitably efficient. I tested it with my own repository and performance seemed fine to me. Especially with goblet's caching, it shouldn't be an issue.

(Note this is the second iteration. The first purely relied on pygit2, but it was too slow.)