path/filepath: in Rel use case-insensitive comparison on Windows

Compare basepath and targetpath using strings.EqualFold.  The absence
of this on Windows causes an unterminating condition in `for` statement
later in the function.

Fixes #13258

Change-Id: Ib5a0caba864ee425dc75ece47b9cf6fb626f47f1
Reviewed-on: https://go-review.googlesource.com/16857
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/path/filepath/path.go

@@ -258,7 +258,7 @@ func Rel(basepath, targpath string) (string, error) {
 	targVol := VolumeName(targpath)
 	base := Clean(basepath)
 	targ := Clean(targpath)
-	if targ == base {
+	if sameWord(targ, base) {
 		return ".", nil
 	}
 	base = base[len(baseVol):]

src/path/filepath/path_test.go

@@ -1034,6 +1034,8 @@ var winreltests = []RelTests{
 	{`C:\`, `D:\`, `err`},
 	{`C:`, `D:`, `err`},
 	{`C:\Projects`, `c:\projects\src`, `src`},
+	{`C:\Projects`, `c:\projects`, `.`},
+	{`C:\Projects\a\..`, `c:\projects`, `.`},
 }
 
 func TestRel(t *testing.T) {