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/env_windows.go

@@ -0,0 +1,54 @@
+// Copyright 2010 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.
+
+// Windows environment variables.
+
+package windows
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+func Getenv(key string) (value string, found bool) {
+	return syscall.Getenv(key)
+}
+
+func Setenv(key, value string) error {
+	return syscall.Setenv(key, value)
+}
+
+func Clearenv() {
+	syscall.Clearenv()
+}
+
+func Environ() []string {
+	return syscall.Environ()
+}
+
+// Returns a default environment associated with the token, rather than the current
+// process. If inheritExisting is true, then this environment also inherits the
+// environment of the current process.
+func (token Token) Environ(inheritExisting bool) (env []string, err error) {
+	var block *uint16
+	err = CreateEnvironmentBlock(&block, token, inheritExisting)
+	if err != nil {
+		return nil, err
+	}
+	defer DestroyEnvironmentBlock(block)
+	blockp := uintptr(unsafe.Pointer(block))
+	for {
+		entry := UTF16PtrToString((*uint16)(unsafe.Pointer(blockp)))
+		if len(entry) == 0 {
+			break
+		}
+		env = append(env, entry)
+		blockp += 2 * (uintptr(len(entry)) + 1)
+	}
+	return env, nil
+}
+
+func Unsetenv(key string) error {
+	return syscall.Unsetenv(key)
+}