Support polling git repositories for new branches/revisions

This is mostly a workaround for the occasional problems with the guix-commits
mailing list, as it can break and then the data service doesn't learn about
new revisions until the problem is fixed.

I think it's still a generally good feature though, and allows deploying the
data service without it consuming emails to learn about new revisions, and is
a step towards integrating some kind of way of notifying the data service to
poll.

scripts/guix-data-service.in

@@ -37,6 +37,8 @@
              (guix-data-service config)
              (guix-data-service database)
              (guix-data-service substitutes)
+             (guix-data-service poll-git-repository)
+             (guix-data-service model git-repository)
              (guix-data-service model guix-revision-package-derivation)
              (guix-data-service web server)
              (guix-data-service web controller)
@@ -204,12 +206,24 @@
 
       (start-substitute-query-threads)
 
-      (when (assoc-ref opts 'update-database)
-        (call-with-new-thread
-         (lambda ()
-           (run-sqitch)
+      (call-with-new-thread
+       (lambda ()
+         (run-sqitch)
 
-           (atomic-box-set! startup-completed #t))))
+         (for-each
+          (lambda (git-repository-details)
+            (when (fifth git-repository-details)
+              (simple-format #t "starting thread to poll ~A (~A)\n"
+                             (second git-repository-details)
+                             (third git-repository-details))
+
+              (start-thread-to-poll-git-repository
+               (first git-repository-details))))
+          (with-postgresql-connection
+           "poll-startup"
+           all-git-repositories))
+
+         (atomic-box-set! startup-completed #t)))
 
       ;; Provide some visual space between the startup output and the
       ;; server starting