Fix formatting pre-commit check

[tikue]

The hashing didn't work, so I replaced it with write-mode=diff.

I also print the diff if the formatting check fails. Example:

$ git commit -am "Test commit"
[PRECOMMIT] Checking that all filenames are ascii ... ok
[PRECOMMIT] Checking for bad whitespace ... ok
[PRECOMMIT] Checking for rustfmt ... ok
[PRECOMMIT] Checking for shasum ... ok
[PRECOMMIT] Checking formatting ... FAILED
Diff of tarpc/src/protocol/mod.rs:


Diff at line 19:
 pub use self::client::{Client, Future};⏎
 pub use self::server::{Serve, ServeHandle};⏎
 ⏎
-pub mod test_fmt { }⏎
+pub mod test_fmt {}⏎
 ⏎
 /// Client errors that can occur during rpc calls⏎
 #[derive(Debug, Clone)]⏎