The following properties of a path are now checked for:
- under top-level .hg
- starts at the root of a windows drive
- contains ".."
- traverses a symlink (e.g. a/symlink_here/b)
- inside a nested repository
If any of these is true, the path is rejected.
The check for traversing a symlink is arguably stricter than necessary;
perhaps we should be checking for symlinks that point outside the
repository.
