fix(git-changed): accept a bare $GITHUB_BASE_REF branch before the default branch

packages/git-changed/README.md

@@ -165,7 +165,7 @@ Same options; returns `result.paths`.
 In order, first hit wins:
 
 1. `options.base` / `--base <ref>` — explicit always wins.
-2. `$GITHUB_BASE_REF` as `origin/<ref>`, **if that ref exists locally**. On a pull request GitHub sets this to the target branch. The existence check matters: without it you hand back a ref that every subsequent git call rejects.
+2. `$GITHUB_BASE_REF` — the PR's target branch — as `origin/<ref>`, else the bare `<ref>`, **whichever exists locally**. `origin/` comes first because Actions fetches only the remote ref and a stale local branch of the same name would diff against the wrong commit; the bare branch covers running the same tool outside CI. Checking existence matters: without it you hand back a ref that every subsequent git call rejects.
 3. The repository default branch — `origin/HEAD` if set, else the first of `origin/main`, `origin/master`, `main`, `master` that resolves.
 
 Then the diff is taken from `git merge-base HEAD <base>`, not from the base tip, so work that landed on the base after you forked is not attributed to you.
@@ -284,7 +284,7 @@ if (process.env.CI && source === 'worktree') {
 | Files from someone else's merge included | You diffed the base tip somewhere else in your pipeline; this package uses the merge base. Verify with `--json` and look at `mergeBase`. |
 | A new module's files are missing | You're not using this package, or not `-uall`. Untracked files inside a new directory need it. |
 | Tool crashes on a missing path | Something passed `existingOnly: false`. The default drops deleted paths. |
-| `origin/main` not found | `$GITHUB_BASE_REF` was set but unfetched. This package falls through to the local default branch rather than returning a broken ref. |
+| `origin/main` not found | `$GITHUB_BASE_REF` was set but unfetched. This package falls back to the local branch of that name, then to the default branch, rather than returning a broken ref. |
 
 ## License
 

packages/git-changed/__tests__/changed.test.ts

@@ -168,14 +168,24 @@ describe('changedFiles', () => {
     expect(result.files.map((f) => f.relative)).toEqual(['dirty.sql']);
   });
 
-  it('ignores $GITHUB_BASE_REF when the remote ref is missing', () => {
+  it('accepts a bare $GITHUB_BASE_REF branch when the remote ref is missing', () => {
     const dir = track(makeRepo());
     process.env.GITHUB_BASE_REF = 'main';
     write(dir, 'dirty.sql');
 
     // `origin/main` does not exist here. A naive implementation hands back that
-    // ref anyway and every later git call rejects it; this falls through to the
-    // local default branch instead.
+    // ref anyway and every later git call rejects it; this falls back to the
+    // local branch of the same name.
+    const result = changedFiles({ cwd: dir });
+    expect(result.base).toBe('main');
+    expect(result.files.map((f) => f.relative)).toEqual(['dirty.sql']);
+  });
+
+  it('falls through to the default branch when $GITHUB_BASE_REF names nothing', () => {
+    const dir = track(makeRepo());
+    process.env.GITHUB_BASE_REF = 'release/9.9';
+    write(dir, 'dirty.sql');
+
     const result = changedFiles({ cwd: dir });
     expect(result.base).toBe('main');
     expect(result.files.map((f) => f.relative)).toEqual(['dirty.sql']);

packages/git-changed/src/base.ts

@@ -25,8 +25,8 @@ export function defaultBranch(cwd: string): string | undefined {
  *
  * 1. an explicit base (`--base`, or `base` in code) — always wins;
  * 2. `$GITHUB_BASE_REF` — the PR's target branch, set by GitHub Actions on
- *    `pull_request` events, as `origin/<branch>` since only the remote ref is
- *    fetched;
+ *    `pull_request` events — as `origin/<branch>`, or the bare branch when the
+ *    remote ref was never fetched;
  * 3. the repository's default branch.
  *
  * `undefined` means "no base is available" — a detached checkout, a fresh repo
@@ -37,12 +37,17 @@ export function defaultBranch(cwd: string): string | undefined {
 export function resolveBase(base?: string, cwd: string = process.cwd()): string | undefined {
   if (base && base.trim()) return base.trim();
 
-  const prBase = process.env.GITHUB_BASE_REF;
-  if (prBase && prBase.trim()) {
-    const ref = `origin/${prBase.trim()}`;
-    if (tryGit(['rev-parse', '--verify', '--quiet', ref], cwd) !== undefined) return ref;
-    // The remote ref is missing (a single-branch or shallow fetch). Fall through
-    // rather than handing back a ref that every later git call will reject.
+  const prBase = process.env.GITHUB_BASE_REF?.trim();
+  if (prBase) {
+    // `origin/<branch>` first: in Actions only the remote ref is fetched, and a
+    // stale local branch of the same name would diff against the wrong commit.
+    // A bare local branch is the fallback for a developer running the same tool
+    // outside CI with the variable exported.
+    for (const ref of [`origin/${prBase}`, prBase]) {
+      if (tryGit(['rev-parse', '--verify', '--quiet', ref], cwd) !== undefined) return ref;
+    }
+    // Neither exists (a single-branch or shallow fetch). Fall through rather than
+    // handing back a ref that every later git call will reject.
   }
 
   return defaultBranch(cwd);