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/Microsoft/go-winio/fileinfo.go

@@ -0,0 +1,73 @@
+// +build windows
+
+package winio
+
+import (
+	"os"
+	"runtime"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// FileBasicInfo contains file access time and file attributes information.
+type FileBasicInfo struct {
+	CreationTime, LastAccessTime, LastWriteTime, ChangeTime windows.Filetime
+	FileAttributes                                          uint32
+	pad                                                     uint32 // padding
+}
+
+// GetFileBasicInfo retrieves times and attributes for a file.
+func GetFileBasicInfo(f *os.File) (*FileBasicInfo, error) {
+	bi := &FileBasicInfo{}
+	if err := windows.GetFileInformationByHandleEx(windows.Handle(f.Fd()), windows.FileBasicInfo, (*byte)(unsafe.Pointer(bi)), uint32(unsafe.Sizeof(*bi))); err != nil {
+		return nil, &os.PathError{Op: "GetFileInformationByHandleEx", Path: f.Name(), Err: err}
+	}
+	runtime.KeepAlive(f)
+	return bi, nil
+}
+
+// SetFileBasicInfo sets times and attributes for a file.
+func SetFileBasicInfo(f *os.File, bi *FileBasicInfo) error {
+	if err := windows.SetFileInformationByHandle(windows.Handle(f.Fd()), windows.FileBasicInfo, (*byte)(unsafe.Pointer(bi)), uint32(unsafe.Sizeof(*bi))); err != nil {
+		return &os.PathError{Op: "SetFileInformationByHandle", Path: f.Name(), Err: err}
+	}
+	runtime.KeepAlive(f)
+	return nil
+}
+
+// FileStandardInfo contains extended information for the file.
+// FILE_STANDARD_INFO in WinBase.h
+// https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-file_standard_info
+type FileStandardInfo struct {
+	AllocationSize, EndOfFile int64
+	NumberOfLinks             uint32
+	DeletePending, Directory  bool
+}
+
+// GetFileStandardInfo retrieves ended information for the file.
+func GetFileStandardInfo(f *os.File) (*FileStandardInfo, error) {
+	si := &FileStandardInfo{}
+	if err := windows.GetFileInformationByHandleEx(windows.Handle(f.Fd()), windows.FileStandardInfo, (*byte)(unsafe.Pointer(si)), uint32(unsafe.Sizeof(*si))); err != nil {
+		return nil, &os.PathError{Op: "GetFileInformationByHandleEx", Path: f.Name(), Err: err}
+	}
+	runtime.KeepAlive(f)
+	return si, nil
+}
+
+// FileIDInfo contains the volume serial number and file ID for a file. This pair should be
+// unique on a system.
+type FileIDInfo struct {
+	VolumeSerialNumber uint64
+	FileID             [16]byte
+}
+
+// GetFileID retrieves the unique (volume, file ID) pair for a file.
+func GetFileID(f *os.File) (*FileIDInfo, error) {
+	fileID := &FileIDInfo{}
+	if err := windows.GetFileInformationByHandleEx(windows.Handle(f.Fd()), windows.FileIdInfo, (*byte)(unsafe.Pointer(fileID)), uint32(unsafe.Sizeof(*fileID))); err != nil {
+		return nil, &os.PathError{Op: "GetFileInformationByHandleEx", Path: f.Name(), Err: err}
+	}
+	runtime.KeepAlive(f)
+	return fileID, nil
+}