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)

rust/hg-direct-ffi/rustfmt.toml

@@ -0,0 +1,3 b''
+max_width = 79
+wrap_comments = true
+error_on_line_overflow = true