godoc: fix writeFileAtomically utility function

If the filename was absolute, writeFileAtomically
used the wrong filename for ioutil.TempFile leading
to non-existent directories and the TempFile would
fail.

R=r, r2
CC=golang-dev
https://golang.org/cl/4224043

src/cmd/godoc/utils.go

@@ -95,7 +95,7 @@ func canonicalizePaths(list []string, filter func(path string) bool) []string {
 // atomically renames that file to the file named by filename.
 //
 func writeFileAtomically(filename string, data []byte) os.Error {
-	f, err := ioutil.TempFile(cwd, filename)
+	f, err := ioutil.TempFile(pathutil.Split(filename))
 	if err != nil {
 		return err
 	}