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/dh/x448/curve_amd64.go

@@ -0,0 +1,30 @@
+//go:build amd64 && !purego
+// +build amd64,!purego
+
+package x448
+
+import (
+	fp "github.com/cloudflare/circl/math/fp448"
+	"golang.org/x/sys/cpu"
+)
+
+var hasBmi2Adx = cpu.X86.HasBMI2 && cpu.X86.HasADX
+
+var _ = hasBmi2Adx
+
+func double(x, z *fp.Elt)             { doubleAmd64(x, z) }
+func diffAdd(w *[5]fp.Elt, b uint)    { diffAddAmd64(w, b) }
+func ladderStep(w *[5]fp.Elt, b uint) { ladderStepAmd64(w, b) }
+func mulA24(z, x *fp.Elt)             { mulA24Amd64(z, x) }
+
+//go:noescape
+func doubleAmd64(x, z *fp.Elt)
+
+//go:noescape
+func diffAddAmd64(w *[5]fp.Elt, b uint)
+
+//go:noescape
+func ladderStepAmd64(w *[5]fp.Elt, b uint)
+
+//go:noescape
+func mulA24Amd64(z, x *fp.Elt)