Support per repository configuration for fetching substitutes

This is so that data.qa.guix.gnu.org can be configured only to query the
branches from the main repository.

guix-data-service/model/git-repository.scm

@@ -22,6 +22,7 @@
   #:use-module (guix-data-service model utils)
   #:export (all-git-repositories
             select-git-repository
+            git-repository-query-substitutes?
             git-repository-id->url
             select-includes-and-excluded-branches-for-git-repository
             count-git-repositories-with-x-git-repo-header-values
@@ -57,6 +58,14 @@
            cgit_url_base
            (string=? fetch_with_authentication "t")))))
 
+(define (git-repository-query-substitutes? conn id)
+  (match (exec-query
+          conn
+          "SELECT query_substitutes FROM git_repositories WHERE id = $1"
+          (list (simple-format #f "~A" id)))
+    (((query_substitutes))
+     (string=? query_substitutes "t"))))
+
 (define (git-repository-id->url conn id)
   (match
       (exec-query