dirs: speed up by storing number of direct children per dir The Python version of the dirs type stores only the number of direct children associated with each directory. That means that while adding a directory, it only has to walk backwards until it runs into a directory that is already in its map. The C version walks all the way to the top-most directory. By copying the Python version's clever trick to the C code, we can speed it up quite a bit. On the Firefox repo, perfdirs now runs in 0.031390, from 0.056518 before the undoing Sid's optimization in the previous change, and 0.061835 before previous his optimization. More practically, it speeds up 'hg status nonexistent' on the Firefox repo from 0.176s to 0.155s. It's unclear why the C version did not have the same cleverness implemented from the start, especially given that they were both written by the same person (Bryan O'Sullivan) very close in time: 856960173630 (scmutil: add a dirs class, 2013-04-10) 02ee846b246a (scmutil: rewrite dirs in C, use if available, 2013-04-10)