git/commit: re-implement submodules file reader (#8438)

Reimplement the submodules parser to not depend on the go-git dependency.

See #8222 for the full refactor context.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8438
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: oliverpool <git@olivier.pfad.fr>
Co-committed-by: oliverpool <git@olivier.pfad.fr>

.golangci.yml

@@ -42,6 +42,8 @@ linters:
               desc: do not use the ini package, use gitea's config system instead
             - pkg: github.com/minio/sha256-simd
               desc: use crypto/sha256 instead, see https://codeberg.org/forgejo/forgejo/pulls/1528
+            - pkg: github.com/go-git/go-git
+              desc: use forgejo.org/modules/git instead, see https://codeberg.org/forgejo/forgejo/pulls/4941
     gocritic:
       disabled-checks:
         - ifElseChain