Partition the package_derivations_by_guix_revision_range table

And create a proper git_branches table in the process.

I'm hoping this will help with slow deletions from the
package_derivations_by_guix_revision_range table in the case where there are
lots of branches, since it'll separate the data for one branch from another.

These migrations will remove the existing data, so
rebuild-package-derivations-table will currently need manually running to
regenerate it.

sqitch/verify/git_branch_id.sql

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

sqitch/verify/partition_package_derivations_by_guix_revision_range.sql

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