Set `in_progress_merge_commit_sha` for fast-forward merges.

1. This flag is used in conjunction with the "Developers can Merge"
   branch permission, to allow a user to merge MRs without push access
   to a branch.

2. Without this flag set, the user doesn't have access to the protected
   branch, and the merge fails.

3. This flag was previously set for the `Repository#merge` method, but
   not the `Repository#ff_merge` method.

app/models/repository.rb

@@ -899,7 +899,7 @@ class Repository
     end
   end
 
-  def ff_merge(user, source, target_branch, options = {})
+  def ff_merge(user, source, target_branch, merge_request: nil)
     our_commit = rugged.branches[target_branch].target
     their_commit =
       if source.is_a?(Gitlab::Git::Commit)
@@ -911,7 +911,10 @@ class Repository
     raise "Invalid merge target" if our_commit.nil?
     raise "Invalid merge source" if their_commit.nil?
 
-    commit_with_hooks(user, target_branch) { their_commit.oid }
+    commit_with_hooks(user, target_branch) do
+      merge_request.update(in_progress_merge_commit_sha: their_commit.oid) if merge_request
+      their_commit.oid
+    end
   end
 
   def merge(user, merge_request, options = {})

app/services/merge_requests/ff_merge_service.rb

@@ -9,7 +9,12 @@ module MergeRequests
     private
 
     def commit
-      repository.ff_merge(current_user, merge_request.diff_head_sha, merge_request.target_branch)
+      repository.ff_merge(current_user,
+                          merge_request.diff_head_sha,
+                          merge_request.target_branch,
+                          merge_request: merge_request)
+    ensure
+      merge_request.update(in_progress_merge_commit_sha: nil)
     end
   end
 end

spec/models/repository_spec.rb

@@ -721,6 +721,30 @@ describe Repository, models: true do
     end
   end
 
+  describe '#ff_merge' do
+    it 'merges the code and return the commit id' do
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
+      merge_commit_id = repository.ff_merge(user,
+                                            merge_request.diff_head_sha,
+                                            merge_request.target_branch,
+                                            merge_request: merge_request)
+      merge_commit = repository.commit(merge_commit_id)
+
+      expect(merge_commit).to be_present
+      expect(repository.blob_at(merge_commit.id, 'files/ruby/feature.rb')).to be_present
+    end
+
+    it 'sets the `in_progress_merge_commit_sha` flag for the given merge request' do
+      merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
+      merge_commit_id = repository.ff_merge(user,
+                                            merge_request.diff_head_sha,
+                                            merge_request.target_branch,
+                                            merge_request: merge_request)
+
+      expect(merge_request.in_progress_merge_commit_sha).to eq(merge_commit_id)
+    end
+  end
+
   describe '#revert' do
     let(:new_image_commit) { repository.commit('33f3729a45c02fc67d00adb1b8bca394b0e761d9') }
     let(:update_image_commit) { repository.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }