A diff look names a file as the repository does: git's side prefix dropped by git's rules, a plain diff's names as written

In a git section (one with a `diff --git` line) the one-letter side prefix
git writes, `a/`/`b/` or with diff.mnemonicPrefix `c/`, `i/`, `w/`, `o/`,
is dropped from its paths only when both paths of that line carry one,
different ones: `--no-prefix` writes none, so its `a/x a/x` is a real
directory `a` and is kept. A plain `diff -u` never strips: its names are
used as written, past the timestamp. `/dev/null` stays itself.

The walker does this once (diff.Walk.side, diff.gitHeader), so the path a
look resolves, and the one a miss says, are the repository's name:
`Look: src/x.zig: no such file here`, `Look: lib/old.zig: deleted by this
diff`. The look no longer tries a stripped name beside the written one.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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