Try canonicalisation when converting File to Dir

[Frederick888]

Without this canonicalisation, if the directory is a symbolic link to
another directory, later at reorient_target_path [1], File and path
argument look like:

- self: File {
    name: "."
    ext: None
    path: "/tmp/test2"
    metadata: {...}
    parent_dir: Some({
      contents: vec![]
      path: "/tmp/test2"
    })
    is_all_all: true
  }
- path: "test1"

...where /tmp/test2 is a symlink to /tmp/test1 and command being used is
exa -laa /tmp/test2.

So the result of dir.join(&*path) becomes /tmp/test2/test1, which
doesn't exist.

The canonicalisation also makes the behaviours of exa -laa /tmp/test2
and exa -laa /tmp/test2/ consistent, where . is not treated as a
symlink in either case. This is a bit different from coreutils ls
though.

Closes #952.

[1]

exa/src/fs/file.rs

Lines 215 to 217 in 42659f9

	else if let Some(dir) = self.parent_dir {
	dir.join(&*path)
	}

[ariasuni]

I have changed the code quite a bit in #872, do you think the two could work together? Also, I think it would be nice to have a test with vagrant (look at the README), do you think you could look at it? Since symlinks are tricky, I think it would be nice to have tests (and check they passes, since they aren’t run on the CI).