Fix duplicate commit comments in the github formatter

[leochab]

Problem

The github formatter posts every offense as a new commit comment on every run, even when nothing changed. On a branch with N offenses, M CI reruns create N×M commit comments. Each one is a separate notification.

Root cause

The read path and the write path for comments use different fields.

Pronto::Github#create_commit_comment sends the diff position using the API's position parameter:

client.create_commit_comment(slug, comment.sha, comment.body,
                             comment.path, nil, comment.position)

Pronto::Github#commit_comments reads the response's line field instead:

Comment.new(sha, comment.body, comment.path, comment.line)

According to the commit-comments API docs, line is closing down. It is not set when a comment is created with position. So reading a comment back returns line: null, with position holding the value instead.

Where this came from

Commit 61beff7 ("compatibility fixes for supporting octokit 8.x", fixing #453) changed the GitHub client from position to line.

This was correct for the pull-request review comments endpoint, where position is closing down and line is current. But the same commit also changed commit_comments, where it is the opposite.

Fix

Read comment.position in commit_comments to match what create_commit_comment writes.

Tests

The first commit updates the github_formatter_spec.rb doubles to use a real diff position. Without the one-line fix, these tests fail; with it, they pass. It also adds a github_spec.rb test that pins which field commit_comments reads.

The second commit is optional. It removes the remaining position: nil values, which can never occur in production, and checks the exact arguments passed to create_commit_comment. This checks that the call is made with the right arguments, not just the right number of times.