Start to handle information about Git branches

Add some new pages /branches and /branch/... as well as a new git_branches
table. Also extend the email processing to enter the branch information in to
the database.

tests/model-git-branch.scm

@@ -0,0 +1,25 @@
+(define-module (test-model-git-branch)
+  #:use-module (srfi srfi-64)
+  #:use-module (guix-data-service database)
+  #:use-module (guix-data-service model git-repository)
+  #:use-module (guix-data-service model git-branch))
+
+(test-begin "test-model-git-branch")
+
+(with-postgresql-connection
+ (lambda (conn)
+   (test-assert "insert-git-branch-entry works"
+     (with-postgresql-transaction
+      conn
+      (lambda (conn)
+        (let* ((url "test-url")
+               (id (git-repository-url->git-repository-id conn url)))
+          (insert-git-branch-entry conn
+                                   "master"
+                                   "test-commit"
+                                   id
+                                   (strftime "%c" (gmtime (current-time)))))
+        #t)
+      #:always-rollback? #t))))
+
+(test-end)