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

[petrochenkov]

cg_diff is deprecated now and shows this when run:

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.