filepath/path: fix Rel buffer sizing

Fixes #2493.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5433079

src/pkg/path/filepath/path.go

@@ -312,7 +312,11 @@ func Rel(basepath, targpath string) (string, error) {
 	if b0 != bl {
 		// Base elements left. Must go up before going down.
 		seps := strings.Count(base[b0:bl], string(Separator))
-		buf := make([]byte, 3+seps*3+tl-t0)
+		size := 2 + seps*3
+		if tl != t0 {
+			size += 1 + tl - t0
+		}
+		buf := make([]byte, size)
 		n := copy(buf, "..")
 		for i := 0; i < seps; i++ {
 			buf[n] = Separator

src/pkg/path/filepath/path_test.go

@@ -629,6 +629,10 @@ var reltests = []RelTests{
 	{"a/b/../c", "a/b", "../b"},
 	{"a/b/c", "a/c/d", "../../c/d"},
 	{"a/b", "c/d", "../../c/d"},
+	{"a/b/c/d", "a/b", "../.."},
+	{"a/b/c/d", "a/b/", "../.."},
+	{"a/b/c/d/", "a/b", "../.."},
+	{"a/b/c/d/", "a/b/", "../.."},
 	{"../../a/b", "../../a/b/c/d", "c/d"},
 	{"/a/b", "/a/b", "."},
 	{"/a/b/.", "/a/b", "."},
@@ -640,6 +644,10 @@ var reltests = []RelTests{
 	{"/a/b/../c", "/a/b", "../b"},
 	{"/a/b/c", "/a/c/d", "../../c/d"},
 	{"/a/b", "/c/d", "../../c/d"},
+	{"/a/b/c/d", "/a/b", "../.."},
+	{"/a/b/c/d", "/a/b/", "../.."},
+	{"/a/b/c/d/", "/a/b", "../.."},
+	{"/a/b/c/d/", "/a/b/", "../.."},
 	{"/../../a/b", "/../../a/b/c/d", "c/d"},
 	{".", "a/b", "a/b"},
 	{".", "..", ".."},