Ninja mkdir elision doesn't handle non-canonical paths

I'll take a look at this next week, but logging this before I forget:

```
test: a/b/../c

a/b/../c:
@mkdir -p $(dir $@)
touch a/b/../c
```

Ninja canonicalizes the path before making the directories, so only `a` is created. In make, `mkdir -p a/b/../c` would create `a` and `a/b`.