cmd/gg: use Git Bash for Windows editor shell command

Updates #47

cmd/gg/editor.go

@@ -61,7 +61,7 @@ func (e *editor) open(ctx context.Context, basename string, initial []byte) ([]b
 	if err := ioutil.WriteFile(path, initial, 0600); err != nil {
 		return nil, fmt.Errorf("open editor: %w", err)
 	}
-	c, err := shellCommand(string(editor) + " " + escape.Shell(path))
+	c, err := shellCommand(e.git.Exe(), string(editor)+" "+escape.Shell(path))
 	if err != nil {
 		return nil, fmt.Errorf("open editor: %w", err)
 	}

cmd/gg/editor_unix.go

@@ -20,6 +20,6 @@ package main
 
 import "os/exec"
 
-func shellCommand(line string) (*exec.Cmd, error) {
+func shellCommand(gitExe, line string) (*exec.Cmd, error) {
 	return exec.Command("/bin/sh", "-c", line), nil
 }

cmd/gg/editor_windows.go

@@ -18,19 +18,19 @@ package main
 
 import (
 	"os/exec"
-
-	"golang.org/x/sys/windows"
+	"path/filepath"
 )
 
-func shellCommand(line string) (*exec.Cmd, error) {
-	cmd, err := exec.LookPath("cmd")
-	if err != nil {
+func shellCommand(gitExe, line string) (*exec.Cmd, error) {
+	// Git for Windows comes with an MSYS2 bash emulation that Git uses to invoke
+	// shell commands. To preserve the semantics of the Git editor line, we use
+	// the same shell.
+	//
+	// In the default install location, git.exe lives at C:\Program Files\Git\cmd\git.exe.
+	// bash.exe lives at C:\Program Files\Git\bin\bash.exe and is not on the PATH.
+	bash := filepath.Join(gitExe, "..", "..", "bin", "bash.exe")
+	if _, err := exec.LookPath(bash); err != nil {
 		return nil, err
 	}
-	return &exec.Cmd{
-		Path: cmd,
-		SysProcAttr: &windows.SysProcAttr{
-			CmdLine: "cmd /C " + line,
-		},
-	}, nil
+	return exec.Command(bash, "-c", line), nil
 }

cmd/gg/main_test.go

@@ -235,7 +235,7 @@ var (
 func (env *testEnv) editorCmd(content []byte) (string, error) {
 	cpPathOnce.Do(func() {
 		if runtime.GOOS == "windows" {
-			cpPath, cpPathError = "copy", nil
+			cpPath, cpPathError = "cp", nil
 		} else {
 			cpPath, cpPathError = exec.LookPath("cp")
 		}