path/filepath: include the original paths in error messages

On Windows, Rel emits error messages of the form `Rel: can't make
\windows relative to \windows`. Rather than emitting paths after
stripping volume names, emit the original paths so as to make those of
the form `Rel: can't make d:\windows relative to c:\windows`.  Fixed a
test that expected the error message to emit clean path instead of the
original.

Fixes #13259

Change-Id: I3a9bd5b137205f22794ec8046b4e917ee48cf750
Reviewed-on: https://go-review.googlesource.com/16858
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/path/filepath/example_unix_test.go

@@ -35,7 +35,7 @@ func ExampleRel() {
 	// On Unix:
 	// "/a/b/c": "b/c" <nil>
 	// "/b/c": "../b/c" <nil>
-	// "./b/c": "" Rel: can't make b/c relative to /a
+	// "./b/c": "" Rel: can't make ./b/c relative to /a
 }
 
 func ExampleSplit() {

src/path/filepath/path.go

@@ -270,7 +270,7 @@ func Rel(basepath, targpath string) (string, error) {
 	baseSlashed := len(base) > 0 && base[0] == Separator
 	targSlashed := len(targ) > 0 && targ[0] == Separator
 	if baseSlashed != targSlashed || !sameWord(baseVol, targVol) {
-		return "", errors.New("Rel: can't make " + targ + " relative to " + base)
+		return "", errors.New("Rel: can't make " + targpath + " relative to " + basepath)
 	}
 	// Position base[b0:bi] and targ[t0:ti] at the first differing elements.
 	bl := len(base)
@@ -296,7 +296,7 @@ func Rel(basepath, targpath string) (string, error) {
 		t0 = ti
 	}
 	if base[b0:bi] == ".." {
-		return "", errors.New("Rel: can't make " + targ + " relative to " + base)
+		return "", errors.New("Rel: can't make " + targpath + " relative to " + basepath)
 	}
 	if b0 != bl {
 		// Base elements left. Must go up before going down.