fsinfo: autocorrect "" to "."

Summary:
Without this:

  In [3]: util.getfstype('')
  IOError: [Errno 2] No such file or directory (os error 2)

And there is a code path hitting this:

  File "edenscm/mercurial/util.py", line 1483, in checknlink
    fstype = getfstype(os.path.dirname(testfile))
		# testfile = '.'
	  # os.path.dirname(".") = ""

The old implementation works fine for an empty path:

	In [2]: m.util.getfstype('')
  Out[2]: 'eden'

So let's make the new Rust implementation consistent.

Reviewed By: xavierd

Differential Revision: D20313387

fbshipit-source-id: 258c424a3e8a796d983e20b0d4656e8e3f413706

eden/scm/lib/fsinfo/src/lib.rs

@@ -222,6 +222,13 @@ mod macos {
 pub fn fstype(path: impl AsRef<Path>) -> io::Result<String> {
     let path = path.as_ref();
 
+    // Auto correct an empty path to ".".
+    let path = if path == Path::new("") {
+        Path::new(".")
+    } else {
+        path
+    };
+
     #[cfg(target_os = "linux")]
     {
         return self::linux::fstype(path);