Fixed path check that could end up in infinite loop on Windows

cmd/tree.go

diff --git a/cmd/tree.go b/cmd/tree.go
index dac864b..252405e 100644
--- a/cmd/tree.go
+++ b/cmd/tree.go
@@ -195,7 +195,15 @@
 		abs = cwd
 	}
 	if abs != "." {
-		for wd := abs; wd != "/"; wd = filepath.Dir(wd) {
+
+		// Previously there was a check on the loop that wd := "/". The path
+		// "/" is a POSIX path so this fails on Windows. Now the check is to
+		// make sure the same wd isn't seen twice. When the same wd happens
+		// more than once it's the beginning of looping on the same location
+		// which is the top level.
+		pwd := ""
+		for wd := abs; wd != pwd; wd = filepath.Dir(wd) {
+			pwd = wd
 
 			// Don't look for packages outside the GOPATH
 			// Note, the GOPATH may or may not end with the path separator.