rust: rustfmt config for hg-direct-ffi For now, we're duplicating it, but it would be probably a good idea to use a single one for the whole workspace (would have implications on the other crates as well)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hg-direct-ffi/rustfmt.toml	Mon Oct 15 11:16:12 2018 +0200
@@ -0,0 +1,3 @@
+max_width = 79
+wrap_comments = true
+error_on_line_overflow = true