Create new repositories using a template. (#499)

Setting the default branch breaks on empty repositories. This change
ensures that new repositories are not created empty by setting a
repository template which, ironically, contains an empty commit on
Bloom's default configuration branch.

Because all repositories created to-date do not use this template and
future repositories created via import will also not use the template,
we should ignore this state once the repository is created.

00-repositories.tf

@@ -211,6 +211,13 @@ resource "github_repository" "repositories" {
   archived     = contains(local._archived_repositories, each.value)
   visibility   = "public"
   has_projects = false
+
+  template {
+    owner                = "ros2-gbp"
+    repository           = "release-repository-template"
+    include_all_branches = true
+  }
+
   lifecycle {
     # Plans that destroy repository releases will delete the repository on
     # GitHub and that shouldn't be done in the normal course of operation.
@@ -225,6 +232,7 @@ resource "github_repository" "repositories" {
       has_issues,
       has_wiki,
       homepage_url,
+      template,
       vulnerability_alerts
     ]
   }

00-ros2_gbp.tf

@@ -25,6 +25,28 @@ resource "github_repository" "dotgithub" {
   }
 }
 
+resource "github_repository" "release-repository-template" {
+  name         = "release-repository-template"
+  visibility   = "public"
+  is_template  = true
+  has_projects = false
+  lifecycle {
+    # Plans that destroy repository releases will delete the repository on
+    # GitHub and that shouldn't be done in the normal course of operation.
+    prevent_destroy = true
+    # Ignore fields that are not set or managed by this terraform project
+    # to prevent unecessary drift.
+    ignore_changes = [
+      allow_merge_commit,
+      description,
+      has_downloads,
+      has_issues,
+      has_wiki,
+      vulnerability_alerts
+    ]
+  }
+}
+
 resource "github_team" "docs_team" {
   name                      = "ros2-gbp-docs"
   description               = "ros2-gbp documentation contributors"