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

Fixes #10802

Compare Volume name and each path elements using strings.EqualFold.

Change-Id: Ibdefdb801d0326e53755bc9cc8c10eed998094e5
Reviewed-on: https://go-review.googlesource.com/16795Reviewed-by: Russ Cox <rsc@golang.org>

src/path/filepath/path.go

@@ -269,7 +269,7 @@ func Rel(basepath, targpath string) (string, error) {
 	// Can't use IsAbs - `\a` and `a` are both relative in Windows.
 	baseSlashed := len(base) > 0 && base[0] == Separator
 	targSlashed := len(targ) > 0 && targ[0] == Separator
-	if baseSlashed != targSlashed || baseVol != targVol {
+	if baseSlashed != targSlashed || !sameWord(baseVol, targVol) {
 		return "", errors.New("Rel: can't make " + targ + " relative to " + base)
 	}
 	// Position base[b0:bi] and targ[t0:ti] at the first differing elements.
@@ -283,7 +283,7 @@ func Rel(basepath, targpath string) (string, error) {
 		for ti < tl && targ[ti] != Separator {
 			ti++
 		}
-		if targ[t0:ti] != base[b0:bi] {
+		if !sameWord(targ[t0:ti], base[b0:bi]) {
 			break
 		}
 		if bi < bl {

src/path/filepath/path_plan9.go

@@ -42,3 +42,7 @@ func join(elem []string) string {
 	}
 	return ""
 }
+
+func sameWord(a, b string) bool {
+	return a == b
+}

src/path/filepath/path_test.go

@@ -1033,6 +1033,7 @@ var winreltests = []RelTests{
 	{`C:a\b\c`, `C:a/b/d`, `..\d`},
 	{`C:\`, `D:\`, `err`},
 	{`C:`, `D:`, `err`},
+	{`C:\Projects`, `c:\projects\src`, `src`},
 }
 
 func TestRel(t *testing.T) {

src/path/filepath/path_unix.go

@@ -44,3 +44,7 @@ func join(elem []string) string {
 	}
 	return ""
 }
+
+func sameWord(a, b string) bool {
+	return a == b
+}

src/path/filepath/path_windows.go

@@ -145,3 +145,7 @@ func joinNonEmpty(elem []string) string {
 func isUNC(path string) bool {
 	return volumeNameLen(path) > 2
 }
+
+func sameWord(a, b string) bool {
+	return strings.EqualFold(a, b)
+}