Inline and remove `process_commit()`.
Inline and remove `process_commit()`.

It has two call sites, but it's pretty simple and its removal makes
things clearer and more consistent: all of the subcommands that call
`bench_commit()` -- `bench_commit`, `bench_local`, `bench_published`,
`process` and `test_benchmarks` -- now do so directly.

nrc/rustc-perf,nrc/rustc-perf,nrc/rustc-perf