git: update gitutil test utilities

- Adds a new GitServeHTTP function to start an http server to serve a
  target git repository.
- Adds a new GitDir helper method to get the path to the .git
  directory
- Updates the GitAdd method to take a variable number of files

Signed-off-by: Justin Chadwell <me@jedevc.com>

util/gitutil/gitutil.go

@@ -6,6 +6,7 @@ import (
 	"net/url"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 
 	"github.com/pkg/errors"
@@ -68,6 +69,14 @@ func (c *Git) RootDir() (string, error) {
 	return c.clean(c.run("rev-parse", "--show-toplevel"))
 }
 
+func (c *Git) GitDir() (string, error) {
+	dir, err := c.RootDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, ".git"), nil
+}
+
 func (c *Git) RemoteURL() (string, error) {
 	// Try to get the remote URL from the origin remote first
 	if ru, err := c.clean(c.run("remote", "get-url", "origin")); err == nil && ru != "" {