Support storing whether to authenticate commits by git repository

To allow using the Guix Data Service when commits aren't authenticated.

sqitch/deploy/git_repositories_add_fetch_with_authentication_field.sql

@@ -0,0 +1,8 @@
+-- Deploy guix-data-service:git_repositories_add_fetch_with_authentication_field to pg
+
+BEGIN;
+
+ALTER TABLE git_repositories
+  ADD COLUMN fetch_with_authentication boolean NOT NULL DEFAULT TRUE;
+
+COMMIT;