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/go-git/go-billy/v5/osfs/os_windows.go

@@ -0,0 +1,61 @@
+// +build windows
+
+package osfs
+
+import (
+	"os"
+	"runtime"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+type fileInfo struct {
+	os.FileInfo
+	name string
+}
+
+func (fi *fileInfo) Name() string {
+	return fi.name
+}
+
+var (
+	kernel32DLL    = windows.NewLazySystemDLL("kernel32.dll")
+	lockFileExProc = kernel32DLL.NewProc("LockFileEx")
+	unlockFileProc = kernel32DLL.NewProc("UnlockFile")
+)
+
+const (
+	lockfileExclusiveLock = 0x2
+)
+
+func (f *file) Lock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	var overlapped windows.Overlapped
+	// err is always non-nil as per sys/windows semantics.
+	ret, _, err := lockFileExProc.Call(f.File.Fd(), lockfileExclusiveLock, 0, 0xFFFFFFFF, 0,
+		uintptr(unsafe.Pointer(&overlapped)))
+	runtime.KeepAlive(&overlapped)
+	if ret == 0 {
+		return err
+	}
+	return nil
+}
+
+func (f *file) Unlock() error {
+	f.m.Lock()
+	defer f.m.Unlock()
+
+	// err is always non-nil as per sys/windows semantics.
+	ret, _, err := unlockFileProc.Call(f.File.Fd(), 0, 0, 0xFFFFFFFF, 0)
+	if ret == 0 {
+		return err
+	}
+	return nil
+}
+
+func rename(from, to string) error {
+	return os.Rename(from, to)
+}