os: add some comments and remove an unused variable in rename func

This slightly clarifies the just-submitted CL 40577.

Updates #19647

Change-Id: I5584ad0e1abbc31796e3e5752351857f2a13d6d7
Reviewed-on: https://go-review.googlesource.com/43625
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/os/file_unix.go

@@ -20,12 +20,14 @@ func fixLongPath(path string) string {
 func rename(oldname, newname string) error {
 	fi, err := Lstat(newname)
 	if err == nil && fi.IsDir() {
-		// if we cannot stat oldname we should
-		// return that error in favor of EEXIST
-		fi, err = Lstat(oldname)
-		if err != nil {
-			if pErr, ok := err.(*PathError); ok {
-				err = pErr.Err
+		// There are two independent errors this function can return:
+		// one for a bad oldname, and one for a bad newname.
+		// At this point we've determined the newname is bad.
+		// But just in case oldname is also bad, prioritize returning
+		// the oldname error because that's what we did historically.
+		if _, err := Lstat(oldname); err != nil {
+			if pe, ok := err.(*PathError); ok {
+				err = pe.Err
 			}
 			return &LinkError{"rename", oldname, newname, err}
 		}