Don't create duplicated functions for code repositories and wiki repositories (#33924)

Fix
https://github.com/go-gitea/gitea/pull/33910#pullrequestreview-2688913865

This PR changed the Repositroy interface in `gitrepo` package which
makes it only focus the relative path in the disk and abstract whether
it's a wiki repository or not.

routers/web/repo/compare.go

@@ -885,7 +885,7 @@ func ExcerptBlob(ctx *context.Context) {
 	gitRepo := ctx.Repo.GitRepo
 	if ctx.Data["PageIsWiki"] == true {
 		var err error
-		gitRepo, err = gitrepo.OpenWikiRepository(ctx, ctx.Repo.Repository)
+		gitRepo, err = gitrepo.OpenRepository(ctx, ctx.Repo.Repository.WikiStorageRepo())
 		if err != nil {
 			ctx.ServerError("OpenRepository", err)
 			return