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.

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

@@ -21,7 +21,8 @@
   #:use-module (squee)
   #:use-module (srfi srfi-19)
   #:use-module (guix-data-service model utils)
-  #:export (insert-git-commit-entry))
+  #:export (insert-git-commit-entry
+            git-commit-exists?))
 
 (define (insert-git-commit-entry conn
                                  git-branch-id
@@ -36,3 +37,11 @@ ON CONFLICT DO NOTHING"
    (list commit
          (number->string git-branch-id)
          (date->string datetime "~s"))))
+
+(define (git-commit-exists? conn commit)
+  (match (exec-query
+          conn
+          "SELECT 1 FROM git_commits WHERE commit = $1"
+          (list commit))
+    (#f #f)
+    (_ #t)))