Fix the 'NULL' values in git_branches for the commits

The git_branches table had 'NULL' values for some commits where the branch was
deleted, importantly this was the string 'NULL', not an actual NULL value.

This commit fixes that, migrating the existing values to be '', and changing
the relevant code.

The primary key is also extended to include the datetime field, as this is
important to allow a branch to be deleted twice.

sqitch/verify/fix_null_values_in_git_branches.sql

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