rename: Fix renaming files in current dir

Don't crash when renaming a file given without a path component (and
without options like -f).

bin/rename

@@ -437,7 +437,9 @@ else:
       continue
 
     for src,dst in ops:
-      os.makedirs(os.path.dirname(dst), exist_ok=True)
+      dstdir=os.path.dirname(dst)
+      if dstdir:
+        os.makedirs(dstdir, exist_ok=True)
       shutil.move(src,dst)
     for src,dst,lnk in linkops:
       os.remove(src)