walk: treat symbolic links to directories as directories

[nikofil]

Due to how walkdir works if symlinks are not followed, symlinks to
directories are seen as simple files by ripgrep. This caused a panic
in some cases due to receiving a WalkEvent::Exit event without a
corresponding WalkEvent::Dir event.

This is fixed by looking at the metadata of the file in the case of a
symlink to determine if it's a directory.

Fixes #1389

[BurntSushi]

Thanks. It's going to take me a bit to review this, as this adds an additional syscall for every symlink that is seen, as far as I can tell. We've had performance problems in the past (especially on Windows) when doing that.

[nikofil]

I see, that could indeed be too much overhead.
The only case that this PR fixes should be the case reported (when the given path is a symlink to a directory). Otherwise when the option to follow symlinks isn't given the depth never changes so there are no WalkEvent::Exit events caused by symlinks:

ripgrep/ignore/src/walk.rs

Lines 1026 to 1030 in 8892bf6

	if depth < self.depth {
	self.depth -= 1;
	self.next = dent;
	return Some(Ok(WalkEvent::Exit));
	}

Therefore checking only the given path, instead of any symlink encountered, should also fix this case. I can try fixing it that way if you agree.

[BurntSushi]

Great, thanks! I think this looks good. I'll be merging it with some tweaks as part of #1486. In particular, I made it so the additional stat check only occurred when the directory entry had depth 0.