git subtree cache

Hi,

I noticed an unexpected behaviour with the cache folder used by git subtree split.

In the cache_setup function a cachedir folder is created and named after the pid of the process, however I don't see that folder being deleted anywhere in the script.

As is I'm seeing the number of cache files constantly growing as new cache folders are created when using split.

Is this an expected behaviour?

Regards, Tomas

```
cache_setup () {
   cachedir="$GIT_DIR/subtree-cache/$$"
   rm -rf "$cachedir" ||
      die "Can't delete old cachedir: $cachedir"
   mkdir -p "$cachedir" ||
      die "Can't create new cachedir: $cachedir"
   mkdir -p "$cachedir/notree" ||
      die "Can't create new cachedir: $cachedir/notree"
   debug "Using cachedir: $cachedir" >&2
}
```