Migrate from `cg_diff` to `cg_annotate --diff`

`cg_diff` is deprecated now and shows this when run:
```rust
Deprecated; use `cg_annotate --diff` instead.
```
`cg_annotate --diff` also produces a better output.

rustc-perf doesn't support older versions of valgrind anyway, so we can probably require a version that supports `cg_annotate --diff`.