Fix line commenting for the initial commit

The initial commit doesn't have a parent, so explicitly pass the blank
SHA and handle that when calculating the position.

CHANGELOG

@@ -7,6 +7,7 @@ v 8.11.0 (unreleased)
   - Fix don't pass a local variable called `i` to a partial. !20510 (herminiotorres)
   - Add delimiter to project stars and forks count (ClemMakesApps)
   - Fix rename `add_users_into_project` and `projects_ids`. !20512 (herminiotorres)
+  - Fix adding line comments on the initial commit to a repo !5900
   - Fix the title of the toggle dropdown button. !5515 (herminiotorres)
   - Rename `markdown_preview` routes to `preview_markdown`. (Christopher Bartz)
   - Update to Ruby 2.3.1. !4948

app/models/commit.rb

@@ -229,7 +229,7 @@ class Commit
 
   def diff_refs
     Gitlab::Diff::DiffRefs.new(
-      base_sha: self.parent_id || self.sha,
+      base_sha: self.parent_id || Gitlab::Git::BLANK_SHA,
       head_sha: self.sha
     )
   end

lib/gitlab/diff/position.rb

@@ -139,13 +139,19 @@ module Gitlab
       private
 
       def find_diff_file(repository)
-        diffs = Gitlab::Git::Compare.new(
-          repository.raw_repository,
-          start_sha,
-          head_sha
-        ).diffs(paths: paths)
+        # We're at the initial commit, so just get that as we can't compare to anything.
+        if Gitlab::Git.blank_ref?(start_sha)
+          compare = Gitlab::Git::Commit.find(repository.raw_repository, head_sha)
+        else
+          compare = Gitlab::Git::Compare.new(
+            repository.raw_repository,
+            start_sha,
+            head_sha
+          )
+        end
+
+        diff = compare.diffs(paths: paths).first
 
-        diff = diffs.first
         return unless diff
 
         Gitlab::Diff::File.new(diff, repository: repository, diff_refs: diff_refs)

spec/lib/gitlab/diff/position_spec.rb

@@ -339,6 +339,48 @@ describe Gitlab::Diff::Position, lib: true do
     end
   end
 
+  describe "position for a file in the initial commit" do
+    let(:commit) { project.commit("1a0b36b3cdad1d2ee32457c102a8c0b7056fa863") }
+
+    subject do
+      described_class.new(
+        old_path: "README.md",
+        new_path: "README.md",
+        old_line: nil,
+        new_line: 1,
+        diff_refs: commit.diff_refs
+      )
+    end
+
+    describe "#diff_file" do
+      it "returns the correct diff file" do
+        diff_file = subject.diff_file(project.repository)
+
+        expect(diff_file.new_file).to be true
+        expect(diff_file.new_path).to eq(subject.new_path)
+        expect(diff_file.diff_refs).to eq(subject.diff_refs)
+      end
+    end
+
+    describe "#diff_line" do
+      it "returns the correct diff line" do
+        diff_line = subject.diff_line(project.repository)
+
+        expect(diff_line.added?).to be true
+        expect(diff_line.new_line).to eq(subject.new_line)
+        expect(diff_line.text).to eq("+testme")
+      end
+    end
+
+    describe "#line_code" do
+      it "returns the correct line code" do
+        line_code = Gitlab::Diff::LineCode.generate(subject.file_path, subject.new_line, 0)
+
+        expect(subject.line_code(project.repository)).to eq(line_code)
+      end
+    end
+  end
+
   describe "#to_json" do
     let(:hash) do
       {