git-codereview: specify --no-track on checkout

My global git config contains

```
[branch]
  autosetupmerge = always
```

As a result, git checkout creates a tracking branch,
so the current branch is master instead of origin/master.
Override the config by specifying --no-track.

Fixes golang/go#9379

Change-Id: I00e0f756606f2b7cb2354cb26e8e58eb7cd33274
Reviewed-on: https://go-review.googlesource.com/23969
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/git-codereview/branch_test.go b/git-codereview/branch_test.go
index c01680f..8d1b2fd 100644
--- a/git-codereview/branch_test.go
+++ b/git-codereview/branch_test.go
@@ -18,7 +18,7 @@
 	checkCurrentBranch(t, "master", "origin/master", false, false, "", "")
 
 	t.Logf("on newbranch")
-	trun(t, gt.client, "git", "checkout", "-b", "newbranch")
+	trun(t, gt.client, "git", "checkout", "--no-track", "-b", "newbranch")
 	checkCurrentBranch(t, "newbranch", "origin/master", true, false, "", "")
 
 	t.Logf("making change")