Switch to storing Git repositories in a table

Rather than just storing the URL in the guix_revisions and
load_new_guix_revision_jobs tables. This will help when storing more
information like tags and branches in the future.

tests/model-git-repository.scm

@@ -0,0 +1,32 @@
+(define-module (test-model-git-repository)
+  #:use-module (srfi srfi-64)
+  #:use-module (guix-data-service database)
+  #:use-module (guix-data-service model git-repository))
+
+(test-begin "test-model-git-repository")
+
+(with-postgresql-connection
+ (lambda (conn)
+   (test-assert "returns an id for a non existent URL"
+     (with-postgresql-transaction
+      conn
+      (lambda (conn)
+        (number?
+         (string->number
+          (git-repository-url->git-repository-id
+           conn
+           "test-non-existent-url"))))
+      #:always-rollback? #t))
+
+   (test-assert "returns the right id for an existing URL"
+     (with-postgresql-transaction
+      conn
+      (lambda (conn)
+        (let* ((url "test-url")
+               (id (git-repository-url->git-repository-id conn url)))
+          (string=?
+           id
+           (git-repository-url->git-repository-id conn url))))
+      #:always-rollback? #t))))
+
+(test-end)