fix(#424): pin diff-local git-error propagation with regression tests

The bug (exit 0 when git diff fails) was introduced in the original
implementation and was already fixed by the executor refactoring, but
with no test coverage to prevent it from regressing.

Makes execCommand a package-level var so tests can inject a stub that
simulates git exit-128 failures without spawning a real git process.
Adds two regression tests:
- GitError_ReturnsError: git fails → diffLocal returns non-nil error
- NoChanges_ReturnsNil:  git succeeds with no output → nil (exit 0)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ako

mdl/executor/cmd_diff_local.go

@@ -753,8 +753,10 @@ func parseRefRange(ref string) (base, target string, isRange bool) {
 	return "", ref, false
 }
 
-// execCommand creates an exec.Cmd for running git commands
-func execCommand(name string, args ...string) *exec.Cmd {
+// execCommand creates an exec.Cmd for running git commands.
+// It is a package-level variable so tests can replace it with a stub that
+// simulates git failures without actually invoking git.
+var execCommand = func(name string, args ...string) *exec.Cmd {
 	return exec.Command(name, args...)
 }
 

mdl/executor/cmd_diff_local_test.go

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package executor
+
+import (
+	"bytes"
+	"context"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/mendixlabs/mxcli/mdl/backend/mock"
+	"github.com/mendixlabs/mxcli/mdl/types"
+)
+
+// gitFailBackend returns an ExecContext backed by a MockBackend that reports
+// MPR v2 with a non-empty ContentsDir — enough to reach findChangedMxunitFiles.
+func gitFailBackend(contentsDir string) *ExecContext {
+	mb := &mock.MockBackend{
+		IsConnectedFunc: func() bool { return true },
+		VersionFunc:     func() types.MPRVersion { return 2 },
+		ContentsDirFunc: func() string { return contentsDir },
+	}
+	return &ExecContext{
+		Context: context.Background(),
+		Backend: mb,
+		Output:  &bytes.Buffer{},
+		Cache:   &executorCache{},
+	}
+}
+
+// TestDiffLocal_GitError_ReturnsError is a regression test for issue #424:
+// when git is not available or the project is not in a git repo, DiffLocal
+// must return a non-nil error so the CLI can exit with code 1.
+//
+// The test replaces execCommand with a stub that simulates "git not a repo"
+// (exit 128) so no real git process is spawned.
+func TestDiffLocal_GitError_ReturnsError(t *testing.T) {
+	orig := execCommand
+	defer func() { execCommand = orig }()
+
+	// Simulate `git diff` failing with exit 128 (not a git repo).
+	execCommand = func(name string, args ...string) *exec.Cmd {
+		// Use the "false" command (always exits 1) or a helper binary that
+		// exits with a specific code. exec.Command("sh", "-c", "exit 128")
+		// produces the same *ExitError that real git produces.
+		return exec.Command("sh", "-c", "exit 128")
+	}
+
+	ctx := gitFailBackend("/tmp/mprcontents")
+	err := diffLocal(ctx, "HEAD", DiffOptions{})
+	if err == nil {
+		t.Fatal("diffLocal must return a non-nil error when git fails (issue #424: exit 0 on git error)")
+	}
+
+	// Error message should mention "git diff" so callers can surface a useful message.
+	if !strings.Contains(err.Error(), "git diff") {
+		t.Errorf("error message should mention 'git diff', got: %q", err.Error())
+	}
+}
+
+// TestDiffLocal_NoChanges_ReturnsNil verifies that when git succeeds but
+// reports no changed files, DiffLocal returns nil (exit 0 is correct).
+func TestDiffLocal_NoChanges_ReturnsNil(t *testing.T) {
+	orig := execCommand
+	defer func() { execCommand = orig }()
+
+	// Simulate `git diff` succeeding with empty output (no changes).
+	execCommand = func(name string, args ...string) *exec.Cmd {
+		return exec.Command("sh", "-c", "exit 0")
+	}
+
+	ctx := gitFailBackend("/tmp/mprcontents")
+	err := diffLocal(ctx, "HEAD", DiffOptions{})
+	if err != nil {
+		t.Errorf("diffLocal must return nil when git reports no changes; got: %v", err)
+	}
+}