filepath: Abs must always return a clean path

When I was first coding Abs, I wondered if people wouldn't
expect the path to be consistently clean, even if the path
passed in was already absolute.

CL 4524078 has a potential problem based on exactly that
assumption, so it feels like this behavior is indeed the
most useful and least surprising.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/4548074

src/pkg/path/filepath/path.go

@@ -247,7 +247,7 @@ func EvalSymlinks(path string) (string, os.Error) {
 // path name for a given file is not guaranteed to be unique.
 func Abs(path string) (string, os.Error) {
 	if IsAbs(path) {
-		return path, nil
+		return Clean(path), nil
 	}
 	wd, err := os.Getwd()
 	if err != nil {

src/pkg/path/filepath/path_test.go

@@ -509,6 +509,7 @@ var abstests = []string{
 
 	// Already absolute
 	"$GOROOT/src/Make.pkg",
+	"$GOROOT/src/../src/Make.pkg",
 }
 
 func TestAbs(t *testing.T) {
@@ -537,5 +538,8 @@ func TestAbs(t *testing.T) {
 		if !filepath.IsAbs(abspath) {
 			t.Errorf("Abs(%q)=%q, not an absolute path", path, abspath)
 		}
+		if filepath.IsAbs(path) && abspath != filepath.Clean(path) {
+			t.Errorf("Abs(%q)=%q, isn't clean", path, abspath)
+		}
 	}
 }