build-info make recipe leaking information about `HEAD`

This makefile recipe is failing because the `git diff` is seeing new information about `HEAD`, as seen in https://github.com/haskell/cabal/actions/runs/8086361638/job/22096588140

https://github.com/haskell/cabal/blob/9e5e815315794d4e5b8e69b36379ac04d0839c53/Makefile#L63-L70

```diff
git diff --exit-code doc/buildinfo-fields-reference.rst
diff --git a/doc/buildinfo-fields-reference.rst b/doc/buildinfo-fields-reference.rst
index 338bbdd..5aed6bf 100644
--- a/doc/buildinfo-fields-reference.rst
+++ b/doc/buildinfo-fields-reference.rst
@@ -1,3 +1,4 @@
+HEAD is now at c0ad47c Go for minimal diff
.. _buildinfo-field-reference:
```