FEATURE: Prefer `origin/main` over `origin/master` if it exists (#89)

This allows us to rename the main branch for plugin repos to
`origin/main` and docker_manager should continue to work.

lib/docker_manager/git_repo.rb

@@ -112,12 +112,29 @@ class DockerManager::GitRepo
     Time.at(unix_timestamp).to_datetime
   end
 
+  def upstream_branch
+    run("for-each-ref --format='%(upstream:short)' $(git symbolic-ref -q HEAD)")
+  end
+
+  def has_origin_main?
+    run("branch -r").include?("origin/main") rescue false
+  end
+
   def tracking_branch
     branch = nil
     if defined?(Discourse.find_compatible_git_resource)
       branch = Discourse.find_compatible_git_resource(path)
     end
-    branch || run("for-each-ref --format='%(upstream:short)' $(git symbolic-ref -q HEAD)")
+    return branch if branch.present?
+
+    head = upstream_branch
+
+    # We prefer `origin/main` to `origin/master`
+    if head == "origin/master"
+      return "origin/main" if has_origin_main?
+    end
+
+    head
   end
 
   def run(cmd)

spec/lib/git_repo_spec.rb

@@ -22,4 +22,28 @@ RSpec.describe DockerManager::GitRepo do
     end
   end
 
+  describe "#branch" do
+
+    it "returns origin/master if a repo hasn't been renamed" do
+      described_class.any_instance.stubs(:upstream_branch).returns("origin/master")
+      described_class.any_instance.stubs(:has_origin_main?).returns(false)
+      repo = described_class.new("dummy", "dummy")
+      expect(repo.branch).to eq("origin/master")
+    end
+
+    it "returns origin/main if a repo has been renamed but still tracks master" do
+      described_class.any_instance.stubs(:upstream_branch).returns("origin/master")
+      described_class.any_instance.stubs(:has_origin_main?).returns(true)
+      repo = described_class.new("dummy", "dummy")
+      expect(repo.branch).to eq("origin/main")
+    end
+
+    it "returns origin/main if a repo points at origin/main" do
+      described_class.any_instance.stubs(:upstream_branch).returns("origin/main")
+      described_class.any_instance.stubs(:has_origin_main?).returns(true)
+      repo = described_class.new("dummy", "dummy")
+      expect(repo.branch).to eq("origin/main")
+    end
+
+  end
 end