Simplify test of refresh that should succeed

shutil.which does not always find the git that Popen would run, at
least on Windows, but it is no worse than running a "command -v" or
"where" command as was previously done here via os.popen.

Author: EliahKagan

Diff for: test/test_git.py

@@ -310,8 +310,7 @@ def test_refresh_bad_git_path(self):
         self.assertRaises(GitCommandNotFound, refresh, "yada")
 
     def test_refresh_good_git_path(self):
-        which_cmd = "where" if os.name == "nt" else "command -v"
-        path = os.popen("{0} git".format(which_cmd)).read().strip().split("\n")[0]
+        path = shutil.which("git")
         refresh(path)
 
     def test_options_are_passed_to_git(self):