When a file was renamed on the target branch, ocr review emitted
'[ocr] WARNING: cannot read file <old path> at ref <to>: exit status 128'.
Two compounding bugs:
1. The parser required 'a/'/'b/' prefixes when matching '--- /dev/null' /
'+++ /dev/null', but git emits these lines without prefixes, so
IsNew/IsDeleted were never set and deleted files fell through to a
doomed 'git show ref:<old path>'.
2. 'rename from' / 'rename to' extended headers were never parsed, and
git diff/show call sites did not force rename detection, so renames
degraded to delete+add whenever the user had diff.renames=false.
Fixes:
- Parse 'rename from'/'rename to', 'new file mode', 'deleted file mode'
and unprefixed /dev/null markers in ParseDiffText.
- Pass --find-renames to all git diff/show invocations so rename
detection no longer depends on user config.
- Add IsRenamed to model.Diff (json: is_renamed) and prefer it in
diffStatus.
- Add parser unit tests and a range-mode rename regression test.
Fixes#99
