Add an index on git_commits

And also remove the duplicates that have crept in.

sqitch/deploy/fix_git_commits_duplicates.sql

@@ -0,0 +1,18 @@
+-- Deploy guix-data-service:fix_git_commits_duplicates to pg
+
+BEGIN;
+
+DELETE FROM git_commits WHERE id IN (
+  SELECT DISTINCT b.id
+  FROM git_commits AS a
+  INNER JOIN git_commits AS b
+  ON a.commit = b.commit AND
+     a.git_branch_id = b.git_branch_id AND
+     a.datetime = b.datetime AND
+     a.id != b.id
+  WHERE a.id < b.id
+);
+
+CREATE UNIQUE INDEX git_commits_unique_idx ON git_commits (git_branch_id, commit, datetime);
+
+COMMIT;

sqitch/revert/fix_git_commits_duplicates.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:fix_git_commits_duplicates from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;

sqitch/sqitch.plan

@@ -86,3 +86,4 @@ package_derivations_not_null_hash_index 2021-07-11T14:19:32Z Christopher Baines
 git_branch_id 2022-05-23T18:11:14Z Chris <chris@felis> # Add git_branch.id
 partition_package_derivations_by_guix_revision_range 2022-05-23T18:20:37Z Chris <chris@felis> # Partition package_derivations_by_guix_revision_range
 package_range_index 2022-06-17T10:39:31Z Chris <chris@felis> # Add index on package_derivations_by_guix_revision_range.package_name
+fix_git_commits_duplicates 2022-06-17T10:39:50Z Chris <chris@felis> # Fix git_commits duplicates

sqitch/verify/fix_git_commits_duplicates.sql

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