cmd/go: fix and reenable TestAccidentalGitCheckout

This is an important security problem so we shouldn't disable the test.
The second half was failing on case-sensitive file systems but the
first half is still good.

Fixes #22983.

Change-Id: I437bb4c9f78eb3177aa8b619e2357b2539566ca9
Reviewed-on: https://go-review.googlesource.com/129797
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/go/go_test.go

@@ -1291,9 +1291,9 @@ func TestGetGitDefaultBranch(t *testing.T) {
 	tg.grepStdout(`\* another-branch`, "not on correct default branch")
 }
 
+// Security issue. Don't disable. See golang.org/issue/22125.
 func TestAccidentalGitCheckout(t *testing.T) {
 	testenv.MustHaveExternalNetwork(t)
-	testenv.SkipFlaky(t, 22983) // this test might not have ever worked; see issue.
 	if _, err := exec.LookPath("git"); err != nil {
 		t.Skip("skipping because git binary not found")
 	}
@@ -1302,13 +1302,17 @@ func TestAccidentalGitCheckout(t *testing.T) {
 	defer tg.cleanup()
 	tg.parallel()
 	tg.tempDir("src")
+
 	tg.setenv("GOPATH", tg.path("."))
 
 	tg.runFail("get", "-u", "vcs-test.golang.org/go/test1-svn-git")
 	tg.grepStderr("src[\\\\/]vcs-test.* uses git, but parent .*src[\\\\/]vcs-test.* uses svn", "get did not fail for right reason")
 
-	tg.runFail("get", "-u", "vcs-test.golang.org/go/test2-svn-git/test2main")
-	tg.grepStderr("src[\\\\/]vcs-test.* uses git, but parent .*src[\\\\/]vcs-test.* uses svn", "get did not fail for right reason")
+	if _, err := os.Stat(tg.path("SrC")); err == nil {
+		// This case only triggers on a case-insensitive file system.
+		tg.runFail("get", "-u", "vcs-test.golang.org/go/test2-svn-git/test2main")
+		tg.grepStderr("src[\\\\/]vcs-test.* uses git, but parent .*src[\\\\/]vcs-test.* uses svn", "get did not fail for right reason")
+	}
 }
 
 func TestErrorMessageForSyntaxErrorInTestGoFileSaysFAIL(t *testing.T) {