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/golang.org/x/sys/windows/race0.go

@@ -0,0 +1,26 @@
+// Copyright 2012 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 windows && !race
+// +build windows,!race
+
+package windows
+
+import (
+	"unsafe"
+)
+
+const raceenabled = false
+
+func raceAcquire(addr unsafe.Pointer) {
+}
+
+func raceReleaseMerge(addr unsafe.Pointer) {
+}
+
+func raceReadRange(addr unsafe.Pointer, len int) {
+}
+
+func raceWriteRange(addr unsafe.Pointer, len int) {
+}