Allow straight diff in Compare API

Repository compare API now allows choosing straight (from..to) or merge-base diff (from...to)

changelogs/unreleased/straight-comparision-mode.yml

+---
+title: Allow straight diff in Compare API
+merge_request: 20120
+author: Maciej Nowak
+type: added

doc/api/repositories.md

@@ -130,6 +130,7 @@ Parameters:
 - `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
 - `from` (required) - the commit SHA or branch name
 - `to` (required) - the commit SHA or branch name
+- `straight` (optional) - comparison method, `true` for direct comparison between `from` and `to` (`from`..`to`), `false` to compare using merge base (`from`...`to`)'. Default is `false`.
 ```
 GET /projects/:id/repository/compare?from=master&to=feature

lib/api/repositories.rb

@@ -100,9 +100,10 @@ module API
       params do
         requires :from, type: String, desc: 'The commit, branch name, or tag name to start comparison'
         requires :to, type: String, desc: 'The commit, branch name, or tag name to stop comparison'
+        optional :straight, type: Boolean, desc: 'Comparison method, `true` for direct comparison between `from` and `to` (`from`..`to`), `false` to compare using merge base (`from`...`to`)', default: false
       end
       get ':id/repository/compare' do
-        compare = Gitlab::Git::Compare.new(user_project.repository.raw_repository, params[:from], params[:to])
+        compare = Gitlab::Git::Compare.new(user_project.repository.raw_repository, params[:from], params[:to], straight: params[:straight])
         present compare, with: Entities::Compare
       end

spec/requests/api/repositories_spec.rb

@@ -288,6 +288,9 @@ describe API::Repositories do
     shared_examples_for 'repository compare' do
       it "compares branches" do
+        expect(::Gitlab::Git::Compare).to receive(:new).with(anything, anything, anything, {
+          straight: false
+        }).and_call_original
         get api(route, current_user), from: 'master', to: 'feature'
         expect(response).to have_gitlab_http_status(200)
@@ -295,6 +298,28 @@ describe API::Repositories do
         expect(json_response['diffs']).to be_present
       end
+      it "compares branches with explicit merge-base mode" do
+        expect(::Gitlab::Git::Compare).to receive(:new).with(anything, anything, anything, {
+          straight: false
+        }).and_call_original
+        get api(route, current_user), from: 'master', to: 'feature', straight: false
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response['commits']).to be_present
+        expect(json_response['diffs']).to be_present
+      end
+
+      it "compares branches with explicit straight mode" do
+        expect(::Gitlab::Git::Compare).to receive(:new).with(anything, anything, anything, {
+          straight: true
+        }).and_call_original
+        get api(route, current_user), from: 'master', to: 'feature', straight: true
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response['commits']).to be_present
+        expect(json_response['diffs']).to be_present
+      end
+
       it "compares tags" do
         get api(route, current_user), from: 'v1.0.0', to: 'v1.1.0'