dangling symlinks cause crash

[wimglenn]

When using PathSpec.match_tree, If there is a broken symlink encountered, we get unhandled exception. It's here:

https://github.com/cpburnz/python-path-specification/blob/da86e2c4d557df2d0a7cc9743268a7173d3a4828/pathspec/util.py#L68

I think you could fix it with os.lstat instead, but that's backwards incompat change.

Perhaps iter_tree could have an option to not follow symlinks? You could follow the example of os.walk, which accepts a followlinks keyword arg, and also an onerror callback which can be used to handle problems such as permission errors when stat each file.

[cpburnz]

I've updated iter_tree to behave similar to os.walk but not identically. Dangling symlinks will now be ignored by default. I've added similar arguments:

• on_error: behaves the same as os.walk.
• follow_links: default is True instead of os.walk's False to maintain backwards compatibility of following links.