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
Introduce gitcmd.Runner with channel-based semaphore to cap concurrent
git subprocesses (default 16, configurable via --max-git-procs). Route
all tool-layer and diff-layer git calls through the shared runner.
Also fix diff.Provider.runGit lacking context.Context — now the full
chain (GetDiff → MergeBase → ParseDiffText → finalizeDiff) propagates
the caller's context for proper cancellation and timeout support.
file_find and finalizeDiff always read from the working tree, even in
range/commit mode where the review targets a specific git ref. This
caused inconsistent file versions compared to file_read and code_search
which correctly used git show. Fix file_find to use git ls-tree and
finalizeDiff to use git show when a ref is specified.
