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.

guix-data-service/model/git-commit.scm

@@ -0,0 +1,38 @@
+;;; Guix Data Service -- Information about Guix over time
+;;; Copyright © 2019 Christopher Baines <mail@cbaines.net>
+;;;
+;;; This program is free software: you can redistribute it and/or
+;;; modify it under the terms of the GNU Affero General Public License
+;;; as published by the Free Software Foundation, either version 3 of
+;;; the License, or (at your option) any later version.
+;;;
+;;; This program is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; Affero General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU Affero General Public
+;;; License along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+(define-module (guix-data-service model git-commit)
+  #:use-module (ice-9 match)
+  #:use-module (json)
+  #:use-module (squee)
+  #:use-module (srfi srfi-19)
+  #:use-module (guix-data-service model utils)
+  #:export (insert-git-commit-entry))
+
+(define (insert-git-commit-entry conn
+                                 git-branch-id
+                                 commit
+                                 datetime)
+  (exec-query
+   conn
+   "
+INSERT INTO git_commits (commit, git_branch_id, datetime)
+VALUES ($1, $2, to_timestamp($3))
+ON CONFLICT DO NOTHING"
+   (list commit
+         (number->string git-branch-id)
+         (date->string datetime "~s"))))