Remove source_branch_name commit check

This should already be done in Gitlab::Git:Compare.

lib/gitlab/git/repository.rb

@@ -733,8 +733,6 @@ module Gitlab
 
       def compare_source_branch(target_branch_name, source_repository, source_branch_name, straight:)
         if source_repository == self
-          return unless commit(source_branch_name).present?
-
           return Gitlab::Git::Compare.new(self, target_branch_name, source_branch_name, straight: straight)
         end
 

spec/lib/gitlab/git/repository_spec.rb

@@ -1980,7 +1980,7 @@ describe Gitlab::Git::Repository, :seed_helper do
       it 'returns nil when source ref does not exist' do
         compare = repository.compare_source_branch('master', repository, 'non-existent-branch', straight: false)
 
-        expect(compare).to be_nil
+        expect(compare.commits).to be_empty
       end
     end