Add a git fetch to the Deploy step of deploy_unified_header.yml.

.github/workflows/deploy_unified_header.yml

@@ -23,6 +23,7 @@ jobs:
     - name: Deploy
       working-directory: ${{github.workspace}}
       run: |
+          git fetch
           git switch single_header
           mv {{github.workspace}}/onebig.hpp ${{github.workspace}}/include/boost/parser/parser_unified.hpp
           git commit -am 'Generated by Github action on push to master.'