Add function getLineDiffDetails

Function for retrieving the details of the diffs in a file. It works the same as `gitLineDiffs`, but uses the `git_diff_line` callback. I extracted the blob retrieval part into its own reusable function.

A package for viewing the diffs inline is already in the works [here](https://github.com/samu/git-diff-details). Needs the changes from [this](https://github.com/samu/atom/compare/update-repo-api) branch.