Fix GH PR discussion snippets filename resolution
Created by: lguychard
Fixes #5728
Fixes filename resolution (and thus code intelligence) for commented snippets on the GitHub.com PR discussion page: these snippets no longer include a .file-info
element.
Updates the code-view.html fixtures for github.com to reflect the new DOM, and adds unit tests for getDeltaFilename()
ensuring that filenames are still correctly resolved for all other types of code views.
Also makes sure to return diffConversationCodeView
for embedded snippets, so that we don't attempt to mount the code view toolbar.