Keep the bounded source prefixes needed to highlight a diff.

Idempotent, and rehydration depends on it: DiffMessage.__init__ calls this on whatever it is handed, which is the full file from the live path but an already-trimmed prefix from MessageData. Re-trimming a prefix must return it unchanged, so any future trimming rule has to stay keyed on the diff's line numbers rather than on a count relative to the input, and has to survive the split/join round trip — see the trailing-newline case in _highlight_source_prefix. test_trimming_a_prefix_again_returns_it_unchanged pins this.