Add . and / to MockFs

Add the final directory to MockFs so that Exists() on . and /
returns true.

Test: java_test.go
Change-Id: I1320f4f267ee6b6a7dbf1132630df306602e84d8
diff --git a/pathtools/fs.go b/pathtools/fs.go
index 4ed91ff..4f98c6e 100644
--- a/pathtools/fs.go
+++ b/pathtools/fs.go
@@ -41,6 +41,7 @@
 			fs.dirs[dir] = true
 			dir = filepath.Dir(dir)
 		}
+		fs.dirs[dir] = true
 	}
 
 	for f := range fs.files {