Fix IDE detecting MR from fork branch

**Why?**
Currently the IDE loads a merge request based on only the
`source_branch` name. This means it loads MR's from
forks that have the same branch name (not good).

- This required updating the BE API to accept `source_project_id`

app/assets/javascripts/ide/stores/actions/merge_request.js

@@ -4,10 +4,11 @@ import service from '../../services';
 import * as types from '../mutation_types';
 import { activityBarViews } from '../../constants';
 
-export const getMergeRequestsForBranch = ({ commit }, { projectId, branchId } = {}) =>
+export const getMergeRequestsForBranch = ({ commit, state }, { projectId, branchId } = {}) =>
   service
     .getProjectMergeRequests(`${projectId}`, {
       source_branch: branchId,
+      source_project_id: state.projects[projectId].id,
       order_by: 'created_at',
       per_page: 1,
     })

app/finders/merge_requests_finder.rb

@@ -40,7 +40,8 @@ class MergeRequestsFinder < IssuableFinder
     items = by_commit(super)
     items = by_source_branch(items)
     items = by_wip(items)
-    by_target_branch(items)
+    items = by_target_branch(items)
+    by_source_project_id(items)
   end
 
   private
@@ -74,6 +75,16 @@ class MergeRequestsFinder < IssuableFinder
     items.where(target_branch: target_branch)
   end
 
+  def source_project_id
+    @source_project_id ||= params[:source_project_id].presence
+  end
+
+  def by_source_project_id(items)
+    return items unless source_project_id
+
+    items.where(source_project_id: source_project_id)
+  end
+
   def by_wip(items)
     if params[:wip] == 'yes'
       items.where(wip_match(items.arel_table))

changelogs/unreleased/ide-fix-detect-mr-from-fork.yml

+---
+title: Fix IDE detection of MR from fork with same branch name
+merge_request: 26986
+author:
+type: fixed

lib/api/merge_requests.rb

@@ -111,6 +111,7 @@ module API
                          desc: 'Return merge requests for the given scope: `created_by_me`, `assigned_to_me` or `all`'
         optional :my_reaction_emoji, type: String, desc: 'Return issues reacted by the authenticated user by the given emoji'
         optional :source_branch, type: String, desc: 'Return merge requests with the given source branch'
+        optional :source_project_id, type: Integer, desc: 'Return merge requests with the given source project id'
         optional :target_branch, type: String, desc: 'Return merge requests with the given target branch'
         optional :search, type: String, desc: 'Search merge requests for text present in the title, description, or any combination of these'
         optional :in, type: String, desc: '`title`, `description`, or a string joining them with comma'

spec/finders/merge_requests_finder_spec.rb

@@ -83,6 +83,14 @@ describe MergeRequestsFinder do
         expect(merge_requests).to contain_exactly(merge_request2)
       end
 
+      it 'filters by source project id' do
+        params = { source_project_id: merge_request2.source_project_id }
+
+        merge_requests = described_class.new(user, params).execute
+
+        expect(merge_requests).to contain_exactly(merge_request1, merge_request2, merge_request3)
+      end
+
       it 'filters by state' do
         params = { state: 'locked' }