cmd/go: accept plain file for .vcs (instead of directory)

Sometimes .git is a plain file; maybe others will follow.
This CL matches CL 21430, made in x/tools/go/vcs.

The change in the Swift test case makes the test case
pass by changing the test to match current behavior,
which I assume is better than the reverse.
(The test only runs locally and without -short, so the
builders are not seeing this particular failure.)

For #10322.

Change-Id: Iccd08819a01c5609a2880b9d8a99af936e20faff
Reviewed-on: https://go-review.googlesource.com/30948
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/vcs.go

@@ -500,7 +500,7 @@ func vcsFromDir(dir, srcRoot string) (vcs *vcsCmd, root string, err error) {
 	origDir := dir
 	for len(dir) > len(srcRoot) {
 		for _, vcs := range vcsList {
-			if fi, err := os.Stat(filepath.Join(dir, "."+vcs.cmd)); err == nil && fi.IsDir() {
+			if _, err := os.Stat(filepath.Join(dir, "."+vcs.cmd)); err == nil {
 				return vcs, filepath.ToSlash(dir[len(srcRoot)+1:]), nil
 			}
 		}

src/cmd/go/vcs_test.go

@@ -102,7 +102,7 @@ func TestRepoRootForImportPath(t *testing.T) {
 			"git.openstack.org/openstack/swift.git",
 			&repoRoot{
 				vcs:  vcsGit,
-				repo: "https://git.openstack.org/openstack/swift",
+				repo: "https://git.openstack.org/openstack/swift.git",
 			},
 		},
 		{
@@ -174,11 +174,23 @@ func TestFromDir(t *testing.T) {
 	}
 	defer os.RemoveAll(tempDir)
 
-	for _, vcs := range vcsList {
+	for j, vcs := range vcsList {
 		dir := filepath.Join(tempDir, "example.com", vcs.name, "."+vcs.cmd)
-		err := os.MkdirAll(dir, 0755)
-		if err != nil {
-			t.Fatal(err)
+		if j&1 == 0 {
+			err := os.MkdirAll(dir, 0755)
+			if err != nil {
+				t.Fatal(err)
+			}
+		} else {
+			err := os.MkdirAll(filepath.Dir(dir), 0755)
+			if err != nil {
+				t.Fatal(err)
+			}
+			f, err := os.Create(dir)
+			if err != nil {
+				t.Fatal(err)
+			}
+			f.Close()
 		}
 
 		want := repoRoot{