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/testutil.go

@@ -39,9 +39,10 @@ func GitCheckoutBranch(c *Git, tb testing.TB, name string) {
 	require.Empty(tb, out)
 }
 
-func GitAdd(c *Git, tb testing.TB, file string) {
+func GitAdd(c *Git, tb testing.TB, files ...string) {
 	tb.Helper()
-	_, err := fakeGit(c, "add", file)
+	args := append([]string{"add"}, files...)
+	_, err := fakeGit(c, args...)
 	require.NoError(tb, err)
 }