path/filepath: return cleaned path from Rel

As the doc says, Rel should return a cleaned path.

Fixes golang#75763

Change-Id: Ic0f5a3b1da3cc4cf3c31fdb1a88ebcc4ea6f9169
Reviewed-on: https://go-review.googlesource.com/c/go/+/709675
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: guerinoni

src/path/filepath/path.go

@@ -248,7 +248,7 @@ func Rel(basepath, targpath string) (string, error) {
 			buf[n] = Separator
 			copy(buf[n+1:], targ[t0:])
 		}
-		return string(buf), nil
+		return Clean(string(buf)), nil
 	}
 	return targ[t0:], nil
 }

src/path/filepath/path_test.go

@@ -1506,6 +1506,7 @@ var reltests = []RelTests{
 	{"/../../a/b", "/../../a/b/c/d", "c/d"},
 	{".", "a/b", "a/b"},
 	{".", "..", ".."},
+	{"", "../../.", "../.."},
 
 	// can't do purely lexically
 	{"..", ".", "err"},