plumbing: use --exec-path to find pack executables

Suggested by smola.

Issue: #527

plumbing/transport/file/client.go

@@ -3,8 +3,11 @@ package file
 
 import (
 	"io"
+	"io/ioutil"
 	"os"
 	"os/exec"
+	"path/filepath"
+	"strings"
 
 	"gopkg.in/src-d/go-git.v4/plumbing/transport"
 	"gopkg.in/src-d/go-git.v4/plumbing/transport/internal/common"
@@ -30,6 +33,30 @@ func NewClient(uploadPackBin, receivePackBin string) transport.Transport {
 	})
 }
 
+func prefixExecPath(cmd string) (string, error) {
+	// Use `git --exec-path` to find the exec path.
+	execCmd := &command{cmd: exec.Command("git", "--exec-path")}
+	err := execCmd.Start()
+	if err != nil {
+		return "", err
+	}
+	err = execCmd.Close()
+	if err != nil {
+		return "", err
+	}
+	stdout, err := execCmd.StdoutPipe()
+	if err != nil {
+		return "", err
+	}
+	execPathBytes, err := ioutil.ReadAll(stdout)
+	if err != nil {
+		return "", err
+	}
+	execPath := string(execPathBytes)
+	execPath = strings.TrimSpace(execPath)
+	return filepath.Join(execPath, cmd), nil
+}
+
 func (r *runner) Command(cmd string, ep transport.Endpoint, auth transport.AuthMethod,
 ) (common.Command, error) {
 
@@ -40,7 +67,17 @@ func (r *runner) Command(cmd string, ep transport.Endpoint, auth transport.AuthM
 		cmd = r.ReceivePackBin
 	}
 
-	return makeCommand(cmd, ep)
+	_, err := exec.LookPath(cmd)
+	if err != nil {
+		if e, ok := err.(*exec.Error); ok && e.Err == exec.ErrNotFound {
+			cmd, err = prefixExecPath(cmd)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+
+	return &command{cmd: exec.Command(cmd, ep.Path())}, nil
 }
 
 type command struct {