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/internal/sha3/keccakf_amd64.go

@@ -0,0 +1,14 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build amd64 && !appengine && !gccgo
+// +build amd64,!appengine,!gccgo
+
+package sha3
+
+// This function is implemented in keccakf_amd64.s.
+
+//go:noescape
+
+func KeccakF1600(state *[25]uint64)