path: Dir

There was Base but not Dir, so fill in the gap.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5504076

src/pkg/path/path.go

@@ -160,3 +160,21 @@ func Base(path string) string {
 func IsAbs(path string) bool {
 	return len(path) > 0 && path[0] == '/'
 }
+
+// Dir returns the all but the last element of path, typically the path's directory.
+// Trailing path separators are removed before processing.
+// If the path is empty, Dir returns ".".
+// If the path consists entirely of separators, Dir returns a single separator.
+// The returned path does not end in a separator unless it is the root directory.
+func Dir(path string) string {
+	dir, _ := Split(path)
+	dir = Clean(dir)
+	last := len(dir) - 1
+	if last > 0 && dir[last] == '/' {
+		dir = dir[:last]
+	}
+	if dir == "" {
+		dir = "."
+	}
+	return dir
+}