add slash_join helper and tests

util/string.py

@@ -0,0 +1,13 @@
+def slash_join(*args):
+  """
+  Joins together strings and guarantees there is only one '/' in between the
+  each string joined. Double slashes ('//') are assumed to be intentional and
+  are not deduplicated.
+  """
+  def rmslash(path):
+    path = path[1:] if path[0] == '/' else path
+    path = path[:-1] if path[-1] == '/' else path
+    return path
+
+  args = [rmslash(path) for path in args]
+  return '/'.join(args)