fix(git): don't treat bare HEAD as a branch diff base

git switch -c / git checkout -b from the current branch record
'branch: Created from HEAD' in the reflog, but parseBranchCreationSource
only rejected 'HEAD@{...}' (detached start) and raw commit hashes. The
bare HEAD passed through, so getBranchBase returned { base: 'HEAD' }
and the branch scope computed diffs against HEAD itself, which is empty
when the service is checked out on that branch and wrong otherwise.

Treat bare HEAD like HEAD@{...}: no named source is recorded, so return
null and let the UI ask the user to pick a base.

packages/web/server/lib/git/service.js

@@ -2660,7 +2660,8 @@ const BRANCH_CREATION_SOURCE_RE = /^branch: Created from (.+)$/;
  * Parse a branch reflog (`git reflog show --format=%gs <branch>`) and return the
  * ref the branch was created from, when that source is itself a named ref.
  *
- * Returns null when the branch was created from `HEAD@{...}` or a raw commit
+ * Returns null when the branch was created from `HEAD` (bare, as `git switch -c`
+ * / `git checkout -b` without an explicit start point record) or a raw commit
  * (detached start): the original branch name is not recorded anywhere in that
  * case, and guessing a base from commit topology would be a heuristic, not an
  * answer. Callers should ask the user to pick a base instead.
@@ -2675,7 +2676,9 @@ export function parseBranchCreationSource(reflogText) {
     const match = lines[index].match(BRANCH_CREATION_SOURCE_RE);
     if (!match) continue;
     const source = match[1].trim();
-    if (!source || /^HEAD@/.test(source) || /^[0-9a-f]{7,40}$/i.test(source)) {
+    // Bare `HEAD` (`git switch -c` from the current branch) and `HEAD@{...}`
+    // (detached start) both lack a named source; a raw commit hash does too.
+    if (!source || /^HEAD(@|$)/.test(source) || /^[0-9a-f]{7,40}$/i.test(source)) {
       return null;
     }
     return source;

packages/web/server/lib/git/service.test.js

@@ -1354,6 +1354,14 @@ describe('parseBranchCreationSource', () => {
     expect(parseBranchCreationSource(reflog)).toBeNull();
   });
 
+  it('returns null when the branch was created from the current HEAD without a named source', () => {
+    // `git switch -c <branch>` / `git checkout -b <branch>` from the current
+    // branch record `branch: Created from HEAD` in the reflog (git 2.x). The
+    // source branch name is not recorded, so no base can be derived from it.
+    const reflog = 'branch: Created from HEAD';
+    expect(parseBranchCreationSource(reflog)).toBeNull();
+  });
+
   it('returns null when the branch was created from a raw commit', () => {
     const reflog = 'branch: Created from 9a3b2c1d4e5f6a7b8c9d0e1f2a3b4c5d6e7f8a9b';
     expect(parseBranchCreationSource(reflog)).toBeNull();