ci: run a pedantic build as part of the GitHub workflow

similar to the recently added sparse task, it is nice to know as early
as possible.

add a dockerized build using fedora (that usually has the latest gcc)
to be ahead of the curve and avoid older ISO C issues at the same time.

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -324,6 +324,8 @@ jobs:
           image: alpine
         - jobname: Linux32
           image: daald/ubuntu32:xenial
+        - jobname: pedantic
+          image: fedora
     env:
       jobname: ${{matrix.vector.jobname}}
     runs-on: ubuntu-latest