diff: when looking for diff configs, support a configurable prefix

Authored by spectral.

Description

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