Introduce find_changed_paths with merge_commit_diff_mode

What does this MR do and why?

Introduce find_changed_paths with merge_commit_diff_mode

Gitaly has implemented the ability to specify how to diff merge commits when requesting a list of changed paths for a list of commits.

FindChangedPaths returns paths that are not cha... (gitaly#4827 - closed)

diff: FindChangedPathsRequest (gitaly!5564 - merged)

After this change we can now pass merge_commit_diff_mode: :all_parents to exclude changes that exist in all parents of the merge commit. This is useful when we have a merge commit that merged changes from base branch back into the current branch.

e.g.

1. User branches from master to new branch named feature/foo_bar
2. User changes ./foo_bar.rb and commits change to feature/foo_bar
3. Another user merges a change to ./bar_baz.rb into master
4. User merges master into feature/foo_bar
5. User pushes to GitLab
6. GitLab checks which files have changed

Related to Git LFS lock in GitLab prevents pushes of new f... (#23625 - closed)

Changelog: added