Change Repository#merge interface for squashing

When squashing, we may want to merge a different commit than the MR's
HEAD commit into the target branch. This interface matches the existing
`Repository#ff_merge` method, and allows us to pick an arbitrary commit
to merge.

app/models/repository.rb

@@ -1027,9 +1027,9 @@ class Repository
     end
   end
 
-  def merge(user, merge_request, options = {})
-    our_commit = rugged.branches[merge_request.target_branch].target
-    their_commit = rugged.lookup(merge_request.diff_head_sha)
+  def merge(user, source, target_branch, merge_request, options = {})
+    our_commit = rugged.branches[target_branch].target
+    their_commit = rugged.lookup(source)
 
     raise "Invalid merge target" if our_commit.nil?
     raise "Invalid merge source" if their_commit.nil?
@@ -1037,7 +1037,7 @@ class Repository
     merge_index = rugged.merge_commits(our_commit, their_commit)
     return false if merge_index.conflicts?
 
-    update_branch_with_hooks(user, merge_request.target_branch) do
+    update_branch_with_hooks(user, target_branch) do
       actual_options = options.merge(
         parents: [our_commit, their_commit],
         tree: merge_index.write_tree(rugged),

app/services/merge_requests/merge_service.rb

@@ -64,7 +64,11 @@ module MergeRequests
         committer: committer
       }
 
-      commit_id = repository.merge(current_user, merge_request, options)
+      commit_id = repository.merge(current_user,
+                                   merge_request.diff_head_sha,
+                                   merge_request.target_branch,
+                                   merge_request,
+                                   options)
 
       if commit_id
         merge_request.update(merge_commit_sha: commit_id)

spec/lib/gitlab/diff/position_tracer_spec.rb

@@ -1640,7 +1640,13 @@ describe Gitlab::Diff::PositionTracer, lib: true do
         }
 
         merge_request = create(:merge_request, source_branch: second_create_file_commit.sha, target_branch: branch_name, source_project: project)
-        repository.merge(current_user, merge_request, options)
+
+        repository.merge(current_user,
+                         merge_request.diff_head_sha,
+                         merge_request.target_branch,
+                         merge_request,
+                         options)
+
         project.commit(branch_name)
       end
 

spec/models/repository_spec.rb

@@ -15,7 +15,13 @@ describe Repository, models: true do
 
   let(:merge_commit) do
     merge_request = create(:merge_request, source_branch: 'feature', target_branch: 'master', source_project: project)
-    merge_commit_id = repository.merge(user, merge_request, commit_options)
+
+    merge_commit_id = repository.merge(user,
+                                       merge_request.diff_head_sha,
+                                       merge_request.target_branch,
+                                       merge_request,
+                                       commit_options)
+
     repository.commit(merge_commit_id)
   end
 
@@ -1009,8 +1015,12 @@ describe Repository, models: true do
 
     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.merge(user, merge_request, commit_options)
-      repository.commit(merge_commit_id)
+
+      merge_commit_id = repository.merge(user,
+                                         merge_request.diff_head_sha,
+                                         merge_request.target_branch,
+                                         merge_request,
+                                         commit_options)
 
       expect(merge_request.in_progress_merge_commit_sha).to eq(merge_commit_id)
     end

spec/services/merge_requests/refresh_service_spec.rb

@@ -101,7 +101,13 @@ describe MergeRequests::RefreshService, services: true do
         # Merge master -> feature branch
         author = { email: 'test@gitlab.com', time: Time.now, name: "Me" }
         commit_options = { message: 'Test message', committer: author, author: author }
-        @project.repository.merge(@user, @merge_request, commit_options)
+
+        @project.repository.merge(@user,
+                                  @merge_request.diff_head_sha,
+                                  @merge_request.target_branch,
+                                  @merge_request,
+                                  commit_options)
+
         commit = @project.repository.commit('feature')
         service.new(@project, @user).execute(@oldrev, commit.id, 'refs/heads/feature')
         reload_mrs