ignore certain files when diffing MPs, configurably

[Paul Collins]

When rendering diffs, GitHub hides (or merely collapses? I didn't click) changes to e.g. *.pb.go, which are typically automatically generated. This seems pretty useful, as we must currently perform certain contortions in order to avoid seeing *.pb.go in MPs when landing changes.

Perhaps some low-risk rules could included by default, although doing this well may require analysis along the lines of "if foo.proto changed, collapse changes to foo.pb.go" since it's not unheard of to have to hack auto-generated files now and then.