Support storing whether to authenticate commits by git repository

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

sqitch/verify/git_repositories_add_fetch_with_authentication_field.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:git_repositories_add_fetch_with_authentication_field on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;