Use winpty to launch fzf in Git bash (mintty)

Close #3806

Known limitation:
* --height cannot be used

src/proxy_windows.go

@@ -0,0 +1,81 @@
+//go:build windows
+
+package fzf
+
+import (
+	"fmt"
+	"io"
+	"os/exec"
+	"strconv"
+	"strings"
+	"sync/atomic"
+)
+
+var shPath atomic.Value
+
+func sh() (string, error) {
+	if cached := shPath.Load(); cached != nil {
+		return cached.(string), nil
+	}
+
+	cmd := exec.Command("cygpath", "-w", "/usr/bin/sh")
+	bytes, err := cmd.Output()
+	if err != nil {
+		return "", err
+	}
+
+	sh := strings.TrimSpace(string(bytes))
+	shPath.Store(sh)
+	return sh, nil
+}
+
+func mkfifo(path string, mode uint32) (string, error) {
+	m := strconv.FormatUint(uint64(mode), 8)
+	sh, err := sh()
+	if err != nil {
+		return path, err
+	}
+	cmd := exec.Command(sh, "-c", fmt.Sprintf(`command mkfifo -m %s %q`, m, path))
+	if err := cmd.Run(); err != nil {
+		return path, err
+	}
+	return path + ".lnk", nil
+}
+
+func withOutputPipe(output string, task func(io.ReadCloser)) error {
+	sh, err := sh()
+	if err != nil {
+		return err
+	}
+	cmd := exec.Command(sh, "-c", fmt.Sprintf(`command cat %q`, output))
+	outputFile, err := cmd.StdoutPipe()
+	if err != nil {
+		return err
+	}
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+
+	task(outputFile)
+	cmd.Wait()
+	return nil
+}
+
+func withInputPipe(input string, task func(io.WriteCloser)) error {
+	sh, err := sh()
+	if err != nil {
+		return err
+	}
+	cmd := exec.Command(sh, "-c", fmt.Sprintf(`command cat - > %q`, input))
+	inputFile, err := cmd.StdinPipe()
+	if err != nil {
+		return err
+	}
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	task(inputFile)
+	inputFile.Close()
+	cmd.Wait()
+	return nil
+}