Update relativize method for java 9

I noticed the io spec was failing on my mac on jdk9 (but passing on
travis, which was running jdk 8). The relativize method arguably didn't
work correctly on pre jdk 9 jres (at least on osx). For example, I ran
the following in the console:

scala> import java.nio.file._

scala> val base = Paths.get("/foo/bar/..")
val base = Paths.get("/foo/bar/..")
base: java.nio.file.Path = /foo/bar/..

scala> val file = Paths.get("/foo/buzz")
val file = Paths.get("/foo/buzz")
file: java.nio.file.Path = /foo/buzz

scala> val relative = base.relativize(file)
val relative = base.relativize(file)
                            ^
relative: java.nio.file.Path = ../../buzz

scala> base.resolve(relative).normalize
base.resolve(relative).normalize
                            ^
res5: java.nio.file.Path = /buzz

This result is certainly not what I would expect. The fix is very easy,
just always normalize the paths (which is not very expensive, especially
compared to any file system operation).

io/src/main/scala/sbt/io/IO.scala

@@ -608,12 +608,14 @@ object IO {
    * If `file` or `base` are not absolute, they are first resolved against the current working directory.
    */
   def relativize(base: File, file: File): Option[String] = {
-    val basePath = (if (base.isAbsolute) base else base.getCanonicalFile).toPath
-    val filePath = (if (file.isAbsolute) file else file.getCanonicalFile).toPath
-    if ((filePath startsWith basePath) || (filePath.normalize() startsWith basePath.normalize())) {
+    val basePath = (if (base.isAbsolute) base else base.getCanonicalFile).toPath.normalize()
+    val filePath = (if (file.isAbsolute) file else file.getCanonicalFile).toPath.normalize()
+    if (filePath startsWith basePath) {
       val relativePath = catching(classOf[IllegalArgumentException]) opt (basePath relativize filePath)
       relativePath map (_.toString)
-    } else None
+    } else {
+      None
+    }
   }
 
   def copy(sources: Traversable[(File, File)]): Set[File] = copy(sources, CopyOptions())

io/src/test/scala/sbt/io/IOSpec.scala

@@ -22,7 +22,7 @@ class IOSpec extends FlatSpec with Matchers {
     val relativeRootDir = new File(nestedDir, "..")
 
     IO.relativize(rootDir.toFile, nestedFile).map(file) shouldBe Some(file("meh.file"))
-    IO.relativize(relativeRootDir, nestedFile).map(file) shouldBe Some(file("../../meh.file"))
+    IO.relativize(relativeRootDir, nestedFile).map(file) shouldBe Some(file("meh.file"))
   }
 
   it should "relativize . dirs" in {