Add merge files files to GetCommitFileStatus (#20515)

Hi,

We'd like to add merge files files to GetCommitFileStatus fucntions so
API returns the list of all the files associated to a merged pull
request commit, like GitHub API does.
The list of affectedFiles for an API commit is fetched from toCommit()
function in routers/api/v1/repo/commits.go, and API was returning no
file in case of a pull request with no conflict, or just files
associated to the confict resolution, but NOT the full list of merged
files.

This would lead to situations where a CI polling a repo for changes
could miss some file changes due to API returning an empty / partial
list in case of such merged pull requests. (Hope this makes sense :) )

NOTE: I'd like to add a unittest in
integrations/api_repo_git_commits_test.go but failed to understand how
to add my own test bare repo so I can make a test on a merged pull
request commit to check for affectedFiles.
Is there a merged pull request in there that I could use maybe?
Could someone please direct me to the relevant ressources with
informations on how to do that please?

Thanks for your time,
Laurent.

---------

Co-authored-by: Thomas Desveaux <desveaux.thomas@gmail.com>

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