feat(tool): add global git subprocess concurrency limiter and propagate context.Context to diff layer

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.

cmd/opencodereview/flags.go

@@ -106,6 +106,7 @@ type reviewOptions struct {
 	concurrency    int
 	perFileTimeout int
 	maxTools       int
+	maxGitProcs    int
 	preview        bool
 	showHelp       bool
 }
@@ -127,6 +128,7 @@ func parseReviewFlags(args []string) (reviewOptions, error) {
 	a.StringVar(&opts.audience, "audience", "human", "output audience: human (show progress) or agent (summary only)")
 	a.StringVarP(&opts.background, "background", "b", "", "optional requirement/business context for the review")
 	a.IntVar(&opts.maxTools, "max-tools", 0, "max tool call rounds per file; only takes effect when greater than template default")
+	a.IntVar(&opts.maxGitProcs, "max-git-procs", 16, "max concurrent git subprocesses")
 	a.BoolVarP(&opts.preview, "preview", "p", false, "preview which files will be reviewed without running the LLM")
 
 	if err := a.Parse(args); err != nil {
@@ -163,6 +165,10 @@ func parseReviewFlags(args []string) (reviewOptions, error) {
 		return opts, fmt.Errorf("--max-tools must be a non-negative integer (0 means use template default)")
 	}
 
+	if opts.maxGitProcs < 0 {
+		return opts, fmt.Errorf("--max-git-procs must be a non-negative integer (0 means use default 16)")
+	}
+
 	return opts, nil
 }
 
@@ -201,6 +207,7 @@ Flags:
   -c, --commit string     single commit hash or tag to review (vs its parent)
   -f, --format string     output format: text or json (default "text")
   --concurrency int       max concurrent file reviews (default 8)
+  --max-git-procs int     max concurrent git subprocesses (default 16)
   --from string           source ref to start diff from (e.g., 'main')
   --max-tools int         max tool call rounds per file; only takes effect when greater than template default
   -p, --preview           preview which files will be reviewed without running the LLM

cmd/opencodereview/review_cmd.go

@@ -12,6 +12,7 @@ import (
 	"github.com/open-code-review/open-code-review/internal/config/template"
 	"github.com/open-code-review/open-code-review/internal/config/toolsconfig"
 	"github.com/open-code-review/open-code-review/internal/diff"
+	"github.com/open-code-review/open-code-review/internal/gitcmd"
 	"github.com/open-code-review/open-code-review/internal/llm"
 	"github.com/open-code-review/open-code-review/internal/stdout"
 	"github.com/open-code-review/open-code-review/internal/telemetry"
@@ -91,6 +92,8 @@ func runReview(args []string) error {
 	llmClient := llm.NewLLMClient(ep)
 	model := ep.Model
 
+	gitRunner := gitcmd.New(opts.maxGitProcs)
+
 	collector := tool.NewCommentCollector()
 	mode := tool.ParseReviewMode(opts.from, opts.to, opts.commit)
 	ref, _ := mode.RefValue(opts.to, opts.commit)
@@ -98,6 +101,7 @@ func runReview(args []string) error {
 		RepoDir: repoDir,
 		Mode:    mode,
 		Ref:     ref,
+		Runner:  gitRunner,
 	}
 	tools := buildToolRegistry(collector, fileReader)
 
@@ -119,6 +123,7 @@ func runReview(args []string) error {
 		ConcurrentTaskTimeout: opts.perFileTimeout,
 		Model:                 model,
 		Background:            opts.background,
+		GitRunner:             gitRunner,
 	})
 
 	// Silence progress output during execution; restore before Summary in agent mode.
@@ -212,15 +217,17 @@ func requireGitRepo(dir string) error {
 }
 
 func runPreview(repoDir string, opts reviewOptions, fileFilter *rules.FileFilter) error {
+	gitRunner := gitcmd.New(opts.maxGitProcs)
 	ag := agent.New(agent.Args{
 		RepoDir:    repoDir,
 		From:       opts.from,
 		To:         opts.to,
 		Commit:     opts.commit,
 		FileFilter: fileFilter,
+		GitRunner:  gitRunner,
 	})
 
-	preview, err := ag.Preview()
+	preview, err := ag.Preview(context.Background())
 	if err != nil {
 		return fmt.Errorf("preview failed: %w", err)
 	}