test: add comprehensive test suite for git.ts lib

- 15 tests covering run(), getBranch(), getStatus(), getRecentCommits(),
  getLastCommit(), getLastCommitTime(), getDiffFiles(), getStagedFiles(),
  getDiffStat()
- Tests use isolated temp git repo with mocked PROJECT_DIR
- Validates no shell injection (literal args, not shell-interpreted)
- Verifies graceful error handling for invalid refs/flags

Author: TerminalGravity

tests/lib/git.test.ts

@@ -0,0 +1,160 @@
+import { describe, it, expect, beforeAll, afterAll, vi } from "vitest";
+import { execFileSync } from "child_process";
+import { mkdtempSync, writeFileSync, rmSync } from "fs";
+import { join } from "path";
+import { tmpdir } from "os";
+
+// We need to mock PROJECT_DIR before importing git functions
+let tempDir: string;
+
+beforeAll(() => {
+  tempDir = mkdtempSync(join(tmpdir(), "preflight-git-test-"));
+
+  // Initialize a git repo with a couple of commits
+  const git = (...args: string[]) =>
+    execFileSync("git", args, { cwd: tempDir, encoding: "utf-8" });
+
+  git("init");
+  git("config", "user.email", "test@test.com");
+  git("config", "user.name", "Test");
+
+  writeFileSync(join(tempDir, "file1.ts"), "export const a = 1;\n");
+  git("add", ".");
+  git("commit", "-m", "initial commit");
+
+  writeFileSync(join(tempDir, "file2.ts"), "export const b = 2;\n");
+  git("add", ".");
+  git("commit", "-m", "second commit");
+});
+
+afterAll(() => {
+  rmSync(tempDir, { recursive: true, force: true });
+});
+
+// Mock PROJECT_DIR to point to our temp repo
+vi.mock("../../src/lib/files.js", () => ({
+  get PROJECT_DIR() {
+    return tempDir;
+  },
+}));
+
+// Import after mock setup (vitest hoists vi.mock, so this works)
+import {
+  run,
+  getBranch,
+  getStatus,
+  getRecentCommits,
+  getLastCommit,
+  getLastCommitTime,
+  getDiffFiles,
+  getStagedFiles,
+  getDiffStat,
+} from "../../src/lib/git.js";
+
+describe("git.run()", () => {
+  it("accepts array args", () => {
+    const result = run(["log", "--oneline", "-1"]);
+    expect(result).toContain("second commit");
+  });
+
+  it("accepts string args (split on whitespace)", () => {
+    const result = run("log --oneline -1");
+    expect(result).toContain("second commit");
+  });
+
+  it("returns error string on invalid command (no throw)", () => {
+    const result = run(["log", "--invalid-flag-xyz"]);
+    expect(result).toMatch(/\[command failed|unknown option|unrecognized/i);
+  });
+
+  it("does NOT interpret shell syntax (no injection)", () => {
+    // If shell were used, this would try to pipe. With execFileSync it's a literal arg.
+    const result = run(["log", "--oneline", "-1", "| cat"]);
+    // Should fail or return an error, not execute "cat"
+    expect(result.startsWith("[") || result.includes("fatal") || result === "").toBe(true);
+  });
+});
+
+describe("getBranch()", () => {
+  it("returns current branch name", () => {
+    const branch = getBranch();
+    // Default branch varies (main/master), just check it's non-empty
+    expect(branch.length).toBeGreaterThan(0);
+  });
+});
+
+describe("getStatus()", () => {
+  it("returns empty string for clean working tree", () => {
+    const status = getStatus();
+    expect(status).toBe("");
+  });
+
+  it("shows modified files", () => {
+    writeFileSync(join(tempDir, "file1.ts"), "export const a = 42;\n");
+    const status = getStatus();
+    expect(status).toContain("file1.ts");
+    // Restore
+    execFileSync("git", ["checkout", "--", "file1.ts"], { cwd: tempDir });
+  });
+});
+
+describe("getRecentCommits()", () => {
+  it("returns requested number of commits", () => {
+    const commits = getRecentCommits(2);
+    const lines = commits.split("\n").filter(Boolean);
+    expect(lines).toHaveLength(2);
+  });
+
+  it("defaults to 5 (returns available)", () => {
+    const commits = getRecentCommits();
+    const lines = commits.split("\n").filter(Boolean);
+    // We only have 2 commits, so should return 2
+    expect(lines.length).toBeLessThanOrEqual(5);
+    expect(lines.length).toBeGreaterThanOrEqual(1);
+  });
+});
+
+describe("getLastCommit()", () => {
+  it("returns single line with commit message", () => {
+    const commit = getLastCommit();
+    expect(commit).toContain("second commit");
+    expect(commit.split("\n")).toHaveLength(1);
+  });
+});
+
+describe("getLastCommitTime()", () => {
+  it("returns ISO-ish timestamp", () => {
+    const time = getLastCommitTime();
+    // Format: 2026-03-09 12:00:00 -0700
+    expect(time).toMatch(/\d{4}-\d{2}-\d{2}/);
+  });
+});
+
+describe("getDiffFiles()", () => {
+  it("returns changed files since ref", () => {
+    const files = getDiffFiles("HEAD~1");
+    expect(files).toContain("file2.ts");
+  });
+
+  it("falls back gracefully for invalid ref", () => {
+    const result = getDiffFiles("nonexistent-ref");
+    // Should fall back to HEAD~1 or return "no commits"
+    expect(typeof result).toBe("string");
+  });
+});
+
+describe("getStagedFiles()", () => {
+  it("returns empty for no staged changes", () => {
+    const staged = getStagedFiles();
+    expect(staged).toBe("");
+  });
+});
+
+describe("getDiffStat()", () => {
+  it("returns stat output for valid ref", () => {
+    const stat = getDiffStat("HEAD~1");
+    expect(stat).toContain("file2.ts");
+    // Stat output includes insertions/deletions
+    expect(stat).toMatch(/\d+ insertion|changed/);
+  });
+});