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;