Switch use libgit2 for diff, remove runtime dependency on git (#20)

There was only one remaining use of the external git dependency in
git-instafix, and it was just to print diffs.

Replacing that with our home-grown git diff code is currently a slightly
worse experience (it doesn't respect user's git diff config) but it is
good enough, and it sets this up for a long term of having significantly
more control.

This also resolves a slight annoyance in the nix package (from [this
comment](#11 (comment)))

Author: quodlibetor

CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+
+* Remove last dependency on external git binary, using libgit2 for all git interactions
+
 # Version 0.2.0
 
 * Rename to git-instafix because there are a bunch of existing projects named git-fixup