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/deploy/git_branch_id.sql

@@ -0,0 +1,32 @@
+-- Deploy guix-data-service:git_branch_id to pg
+
+BEGIN;
+
+ALTER TABLE git_branches RENAME TO git_branches_old;
+
+CREATE TABLE git_branches (
+    id integer PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
+    name character varying NOT NULL,
+    git_repository_id integer NOT NULL REFERENCES git_repositories (id),
+    CONSTRAINT git_repository_id_name_unique UNIQUE (git_repository_id, name)
+);
+
+CREATE TABLE git_commits (
+    id integer PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
+    commit character varying NOT NULL,
+    git_branch_id integer NOT NULL REFERENCES git_branches (id),
+    datetime timestamp without time zone NOT NULL
+);
+
+INSERT INTO git_branches (name, git_repository_id)
+  SELECT DISTINCT name, git_repository_id
+  FROM git_branches_old;
+
+INSERT INTO git_commits (commit, git_branch_id, datetime)
+  SELECT commit, git_branches.id, datetime
+  FROM git_branches_old
+  INNER JOIN git_branches
+    ON git_branches_old.name = git_branches.name
+   AND git_branches_old.git_repository_id = git_branches.git_repository_id;
+
+COMMIT;

sqitch/deploy/partition_package_derivations_by_guix_revision_range.sql

@@ -0,0 +1,18 @@
+-- Deploy guix-data-service:partition_package_derivations_by_guix_revision_range to pg
+
+BEGIN;
+
+DROP TABLE package_derivations_by_guix_revision_range;
+
+CREATE TABLE package_derivations_by_guix_revision_range (
+  git_branch_id integer NOT NULL REFERENCES git_branches (id),
+  package_name varchar NOT NULL,
+  package_version varchar NOT NULL,
+  derivation_id integer NOT NULL,
+  system varchar NOT NULL,
+  target varchar NOT NULL,
+  first_guix_revision_id integer NOT NULL REFERENCES guix_revisions (id),
+  last_guix_revision_id integer NOT NULL REFERENCES guix_revisions (id)
+) PARTITION BY LIST (git_branch_id);
+
+COMMIT;