feat(repo): handle detached HEAD state

Author: VChet

src/index.ts

@@ -1,12 +1,17 @@
 import process from "node:process";
-import { getConfig, getDefaultTargetBranch, getMergedBranches, isGitRepo } from "./repo.js";
+import { getConfig, getDefaultTargetBranch, getMergedBranches, isDetachedHead, isGitRepo } from "./repo.js";
 import { outputMergedBranches } from "./output.js";
 
 function main(): void {
   if (!isGitRepo()) {
     console.error("Not a git repository.");
     process.exit(1);
   }
+  if (isDetachedHead()) {
+    console.error("HEAD is detached (e.g., after checkout of a commit). Please switch to a branch.");
+    process.exit(1);
+  }
+
   const targetBranch = getDefaultTargetBranch();
   if (!targetBranch) {
     console.error("No 'master' or 'main' branch found.");

src/repo.ts

@@ -11,7 +11,16 @@ export function isGitRepo(): boolean {
   }
 }
 
-export function isBranchExists(branch: string): boolean {
+export function isDetachedHead(): boolean {
+  try {
+    const output = execSync("git symbolic-ref --quiet --short HEAD", { encoding: "utf-8" }).trim();
+    return output === "";
+  } catch {
+    return true;
+  }
+}
+
+function isBranchExists(branch: string): boolean {
   try {
     execSync(`git show-ref --verify --quiet refs/heads/${branch}`);
     return true;