* server: skip non-UTF-8 paths in the worktree diff instead of failing it

* server: skip non-UTF-8 paths in the turn diff too

turns::git decoded the whole numstat output as UTF-8 before
numstat_files could skip odd records, so one Latin-1 path failed the
turn-scoped diff with BAD_PARAMS. Read it through a new git_raw and hand
the bytes to numstat_files.

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Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>