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
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