diff: when looking for diff configs, support a configurable prefix In a future commit, I want to make it possible to have the diff options pulled from (as an example) `commands.commit.interactive.ignorews`; previously we only supported this for customizable sections (so this would have needed a `commit-interactive` section and been named `commit-interactive.ignorews`, which felt a bit weird. Differential Revision: https://phab.mercurial-scm.org/D5833