Fix Rel() after ReplaceExtension or InSameDir

Copying p.rel doesn't work, as rel needs to match the end of the
value in path.  Apply the same transformation to p.rel as p.path.

Test: paths_test.go
Change-Id: I42d676c6c4fc18d9852c1a73f25e5a791d7553d0
diff --git a/android/paths_test.go b/android/paths_test.go
index 3b6d2ec..20a00a0 100644
--- a/android/paths_test.go
+++ b/android/paths_test.go
@@ -696,22 +696,26 @@
 	ctx := &configErrorWrapper{
 		config: TestConfig("out", nil),
 	}
-	p := PathForOutput(ctx, "system/framework/boot.art")
+	p := PathForOutput(ctx, "system/framework").Join(ctx, "boot.art")
 	p2 := p.ReplaceExtension(ctx, "oat")
 	fmt.Println(p, p2)
+	fmt.Println(p.Rel(), p2.Rel())
 
 	// Output:
 	// out/system/framework/boot.art out/system/framework/boot.oat
+	// boot.art boot.oat
 }
 
 func ExampleOutputPath_FileInSameDir() {
 	ctx := &configErrorWrapper{
 		config: TestConfig("out", nil),
 	}
-	p := PathForOutput(ctx, "system/framework/boot.art")
+	p := PathForOutput(ctx, "system/framework").Join(ctx, "boot.art")
 	p2 := p.InSameDir(ctx, "oat", "arm", "boot.vdex")
 	fmt.Println(p, p2)
+	fmt.Println(p.Rel(), p2.Rel())
 
 	// Output:
 	// out/system/framework/boot.art out/system/framework/oat/arm/boot.vdex
+	// boot.art oat/arm/boot.vdex
 }