Merge branch '20895-inline-comments-for-first-commit' into 'master'

Fix line commenting for the initial commit

## What does this MR do?

Support line positions on the initial commit, where we can't compare because there's no parent commit.

## Are there points in the code the reviewer needs to double check?

I chose to use the blank SHA to represent the initial commit, but it could as easily be the same SHA. I just thought this was clearer.

## Why was this MR needed?

People couldn't add line comments to the initial commit!

## What are the relevant issue numbers?

Closes #20895.

## Screenshots (if relevant)

![Comment_on_initial_commit](/uploads/1164448462ff55d133f3ff51c98044df/Comment_on_initial_commit.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added (N/A, regression)
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md) (N/A)
- [x] API support added (N/A)
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5900

1 files changed, 12 insertions, 6 deletions

diff --git a/lib/gitlab/diff/position.rb b/lib/gitlab/diff/position.rb
index 2fdcf8d7838..ecf62dead35 100644
--- a/lib/gitlab/diff/position.rb
+++ b/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)