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

config: extract diff-related coreconfigitem()s to a helper method We already have 'annotate' and 'diff' that use the same set of options, and I want to add more in a followup commit, so I'm attempting to reduce maintenance burden and duplication by making it possible to register all of them at once. Differential Revision: https://phab.mercurial-scm.org/D5832