test(git): pin the diff/show/log routing against real git

Four behaviours that fuzzing found and that unit tests over the predicates
could not have caught, because each is about which git child RTK believes
rather than about how one flag classifies: the error `git diff -Uabc
nonexistent-ref` reports, the body surviving `--color` in all three
spellings plus the `color.ui` config form, the log limit announcing itself
only when it took something, and `--quiet` losing to a patch request from
either side while `-s` folds in order.

They run against the real binary in a temp repo with LC_ALL=C pinned, in
CI rather than behind #[ignore], because every one of them was a regression
that the suite as it stood reported as green.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>