R1:f7f3dd5b2084

Don't run Herald build and mail rules when they don't make sense

Summary:
Ref T2543. Fixes T10109.

Currently, Herald only runs in Differential when a change updates the diff. This is partly for historical reasons, and partly because we don't want to restart builds every time someone makes a comment. However, this behavior is inconsistent with other applications (which always trigger on any change), and occasionally confusing to users (in T10109, for example) or otherwise undesirable.

A similar issue is…