feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/cloudflare/circl/sign/ed25519/pubkey112.go

@@ -0,0 +1,7 @@
+//go:build !go1.13
+// +build !go1.13
+
+package ed25519
+
+// PublicKey is the type of Ed25519 public keys.
+type PublicKey []byte