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/sha3_s390x.s

@@ -0,0 +1,33 @@
+// Copyright 2017 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.
+
+// +build !gccgo,!appengine
+
+#include "textflag.h"
+
+// func kimd(function code, chain *[200]byte, src []byte)
+TEXT ·kimd(SB), NOFRAME|NOSPLIT, $0-40
+	MOVD function+0(FP), R0
+	MOVD chain+8(FP), R1
+	LMG  src+16(FP), R2, R3 // R2=base, R3=len
+
+continue:
+	WORD $0xB93E0002 // KIMD --, R2
+	BVS  continue    // continue if interrupted
+	MOVD $0, R0      // reset R0 for pre-go1.8 compilers
+	RET
+
+// func klmd(function code, chain *[200]byte, dst, src []byte)
+TEXT ·klmd(SB), NOFRAME|NOSPLIT, $0-64
+	// TODO: SHAKE support
+	MOVD function+0(FP), R0
+	MOVD chain+8(FP), R1
+	LMG  dst+16(FP), R2, R3 // R2=base, R3=len
+	LMG  src+40(FP), R4, R5 // R4=base, R5=len
+
+continue:
+	WORD $0xB93F0024 // KLMD R2, R4
+	BVS  continue    // continue if interrupted
+	MOVD $0, R0      // reset R0 for pre-go1.8 compilers
+	RET